Analysis of mechanical properties

The tensile mechanical property was tested by an Autograph IS-5000 Mechanical Tester (Shimadzu Co, Japan) with 100 kg load cell. The specimens were rectangular, about 25mm in width and about 70mm in length. The crosshead speed was set at 2.5 mm/min, and the load was applied until

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ultimate fracture of the specimen. The elastic modulus was calculated as the slope of initial linear portion of the stress-strain curve. The tensile strength was determined as the maximum point of the stress-strain curve.

4.3 Results and Discussion

Calcite, Aragonite and Vaterite are isomers of calcium carbonate. The study aims to confirm the lattice structures of commercial calcium carbonate and natural coral reefs. KBr pellets of commercial calcium carbonate and those of natural coral reefs were first placed on KBr Pellet Holder to conduct FTIR spectra analyses. In the meantime, the lattice structure of the white calcium carbonate powder which had been ground in agate mortars and filtered with a 0.45μ filter was also analyzed via XRD spectra. The results are demonstrated in Figure 4-6 The absorption bands chosen for the quantitative analysis of a ternary mixture are the 713 cm^-1 for calcite, the 700 and 712 cm^-1 for aragonite.

Aragonite displays a characteristic symmetric carbonate stretching vibration (ν1) at 1083 cm^-1 and a carbonate out-of plane bending vibration (ν2) at 854 stretching (ν1) vibration is both IR- and Raman-active for aragonite, it is only Raman-active in the case of calcite. Therefore, the peak around 1080 cm^-1 is sometimes used to quantify aragonite from a mixture of aragonite and calcite[58, 89].

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Figure 4- 6 FTIR spectra of calcite and aragonite showing their characteristic carbonate vibrational bands.

Figure 4-7 the XRD spectra of calcite, aragonite are shown in Fig. 4. The area under the principal XRD line at 3.39 Å (2θ: 26.3˚) for aragonite samples.

It is compared with the data from the JCPDS file No. 5-453 and all peaks of the obtained product can be assigned to those of aragonite crystals, indicating the formation of single-phase aragonite. calcite peak (2θ= 29.4˚–29.58˚) and the primary aragonite peak (2θ= 26.28˚). JCPDS file No. 05-0586 and all peaks of the obtained product can be assigned to those of calcite crystals, indicating the formation of single-phase calcite. The calcium planes of calcite are (104) and (113) crystalline plane, while that of aragonite are the (111) and (221). The peaks attributed to the 104, 113 and 111, 221 reflections of calcite and aragonite, respectively, are also used for the quantitative analysis[58, 90-91].

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Figure 4- 7 Powder X-ray diffractograms of calcite and aragonite.

It can be observed from the electronic microscope that, in Figure 4-8(a), the fiber diameter of PLLA is approximately 3~5μm and on the surface of the fibers are pores at the size of around 10nm. In Figure 4-8(b) where 5% of calcium carbonate is added with PLLA, other than the similarities in terms of the size of the fiber diameter (3~5μm) and that of the surface pores (10nm), a 20µm shuttle-like structure comes into being. In Figure 4-8(c) where 5% of coral is added with PLLA, other than the similarities (3~5μm fiber diameter and 10nm surface pores), a 10µm shuttle-like structure is created.

Figure 4- 8 Field Emission Scanning electron micrographs of the electrospinning

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fibers: (a) PLLA fibers, (b) PLLA/calcite fibers(c) PLLA/aragonite fibers image.

Figure 4-9 is Micro-Raman spectra of the synthetically prepared PLLA fibers, PLLA/calcite fibers and PLLA/aragonite fibers. The CH3 asymmetric deformation modes appeare at about 1450±2cm^-1 , the same as intense Raman and IR bands would have detected in all the compounds[77]. The 1250-1400 cm^-1 is region of the Raman spectra. Like in polypropylene and poly(α-L-alanine), this region is characterized by three group of bounds at 1390, 1360 and 1300 cm^-1.C=O stretching region. Presents the 1650-1850 cm^-1 is region of the Raman spectra. Raman spectra (800-950 cm^-1 region) of poly(L-lactic acid)s. Skeletal stretching and rCH3 rocking region in figure 3 shows several strong absorption bands in the 1000-1250 cm^-1 region. The Raman spectrum for the as-prepared sample is shown in Figure 3. Raman spectra can also be used to identify and distinguish aragonite from the other two calcium carbonate phases. The strongest line in each spectrum is theν1 symmetric stretch of CO3

2- ion at ~1080 cm^-1. Raman spectra of calcium carbonate (calcite) and coral (aragonite). The peaks chosen for analysis are the 711 cm^-1 for calcite and 705 cm^-1 for aragonite. The stronger and more highly resolved bands at ~ 1085 cm^-1 are unable to be used in the analysis of the mixed systems due to extensive overlap between the three polymorphs in this region. The Raman spectra of calcite and aragonite are in good agreement with previous reports[58, 92].

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Figure 4- 9 Micro-Raman spectra of the synthetically prepared PLLA fibers, PLLA/calcite fibers and PLLA/aragonite fibers.

As mentioned in previous literature, I875/I1452 Raman intensity ratios can signal results of bio-degradation and structural differences[77]. In this experiment, although both are bio-degradable membranes, the I875/I1452 Raman intensity ratio of PLLA fiber membranes is 1.62, that of calcite-PLLA fiber membranes is 1.62, while that of aragonite-PLLA fiber membranes is 1.57.

RAMAN spectrum is utilized to confirm that calcite and aragonite have been successfully added with PLLA. Also, the I875/I1452 Raman intensity ratios have indicated that they are biodegradable membranes, and Carbonic acid calcium salt, such as calcite and aragonite, does not interfere with the characteristics of PLLA.

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Figure 4- 10 Tensile tests (a) specimen size 25 mm× 70mm× 0.1mm (b) Instrument setup

Figure 4- 11 Stress-strain curves of electrospun of PLLA, PLLA/calcite and PLLA/aragonite fiber.

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Following the above experiment which confirms that the chemical properties of PLLA are not changed with the addition of calcite or aragonite, this experiment would produce calcite-PLLA or aragonite-PLLA membranes and conduct mechanical property analyses where tensile tests would be the main tool. Tensile tests are to measure the resistance capability of material bearing load or incremental load in a stationary status. During the process, both ends of the specimens are held tight and thus under axial stress. As a result, the specimens are stretched longer along axial direction. The specs of the rectangular tensile specimen membranes in this experiment were 25 mm×

70mm× 0.1mm(Figure 4-10). Figure 4-11 demonstrates the stress-strain curves of PLLA, calcite-PLLA, and aragonite-PLLA membranes. Stress value can be derived by dividing load (Newton) by cross-section area (m²), while strain value can be derived by dividing incremental length after stretch by original length. Table 3-1 demonstrates that the 0.2% offset yield strength of PLLA is 1.44MPa, that of PLLA with 5% calcite is 1.68MPa, and that of PLLA with 5% aragonite is 1.94MPa. Therefore, we can tell that PLLA with 5% calcite enjoys a higher 0.2% offset yield strength than pure PLLA by approximately 17％, and PLLA with 5% aragonite enjoys a higher 0.2% offset yield strength than pure PLLA by approximately 35％. Furthermore, the 0.2% offset yield strength of PLLA with 5% aragonite is higher than that of PLLA with 5%

calcite by 18％. The reason is that although calcite and its polymorphism, aragonite, share the same chemical composition, aragonite is formed under higher environment pressure than calcite and could easily transform into calcite with lower energy state under atmospheric pressure (while calcite is

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kT

v Ev

N e

n 

more stable under normal temperature and pressure). Therefore, diagenesis could easily turn aragonite into calcite. Previous studies by Hemming et al.

have found that the anion vacancy of aragonite lattice (37.77ų) is smaller than that of calcite lattice (40.87ų).We can thus tell that aragonite is denser than calcite. Vacancy, a form of lattice imperfection, is categorized as a point defect. A point defect is defined as a departure from symmetry in the alignment of atoms in a lattice structure that affects only one or two lattice sites. It is the most common and important imperfection in lattice. As temperature has a bearing on the presence of vacancy, certain number of vacancy will exist per unit volume under heat balance (T0K). As temperature rises, heat increases, giving atoms enough energy to depart from lattice sites and making it more likely for vacancy to come into existence. The relation is as below:

(1)

n is the number of vacancy, N is the positional number of lattice, Ev is the energy needed to form vacancy, k is Boltzmann constant, while T is absolute temperature. According to the above relation, when temperature rises, vacancy concentration will increase and diffusion will accelerate. However, lattice imperfection usually impacts characteristics of materials, such as mechanical properties, conductivity, photovoltaic properties, magnetism, etc. This is the main factor contributing to the differences in the mechanical properties of guided tissue regeneration membranes under this experiment.

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Table 4- 1: Mechanical properties of the electrospun PLLA, PLLA/calcite and PLLA/aragonite fiber.

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Chapter 5 Ultraviolet Resistant and Degradable Membranes