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Transcript

EFFECTS OF MOLECULAR ORIENTATION AND ANNEALING ON OPTICAL ABSORBTION OF ORIENTED PET POLYMER By Montaser Daraghmeh 1 1.1 Definition Polymers A polymer is made of large giant molecules or chains built up by repetition of small chemical units. The repeated units are called monomers. Polymers are sometimes called "giants" or "macromolecules". 2 1. Introduction: 1.2 Crystalline and Amorphous Polymers When the polymer molecules are arranged in an ordered manner, the polymer is said to be crystalline. Amorphous polymers are those which show no crystalline order when examined by X-rays Lemmellar structure 3 1. Introduction: 1.3 Isotropic and Anisotropic Polymers The isotropic material has the same properties in all directions. Thus, the mechanical properties are independent of direction. In anisotropic materials, the physical properties change with direction. Isotropic structure- a two phase model (crystalline and amorphous). 4 1. Introduction: X-ray photographs: a) Isotropic b) Anisotropic a b Axial drawing axis strain ellipsoid a) Isotropic polymer (circle) b) b) Anisotropic polymer (strain ellipsoid) (a) (b) 5 1. Introduction: 1.5 Orientation of Polymers When the polymer chains in the crystalline regions are aligned to some extent in a certain direction, then the polymer is said to be oriented, and also it becomes anisotropic material. A schematic diagram of polymers: a) Unoriented b) Oriented a b 6 2. Theoretical Background We need the theoretical background to determine the optical properties of polyethylene terephthalate which have the following merits: 2.1 Absorption of light A brief discussion of the absorption process of light near the absorption edge will be presented. This process includes: (1 ) Interband electron absorption. ( 2) Urbach tails 7 2. Theoretical Background Light absorption in a solid * dI ( ) ( ) I ( )dx Integration of above equation gives: I ( ) I o ( ) exp( ( )) x The transmission of a thin layer of fine thickness "dx" can be defined as the ratio between the transmitted and incident light intensities i.e, I ( ) T ( ) e ( ). X I ( ) o 8 2. Theoretical Background general behavior of the absorption coefficient with photon energy is represented by the curve, which shows three main regions: 1. In the low photon energy region of the curve the empirical Urbach rule as: exp .[(w E1 ) / E ] o 2. Intermediate absorption region 3. At high photon energy General shape of the absorption spectrum in a semiconductor 9 2. Theoretical Background Interband transition can take place via two possible mechanisms A) Direct Transition: Light penetrating a sample spends its energy on the excitation of electrons from the valance band to the conduction band. This occurs when * w E g ** k P K e Direct transitions of electrons. Theoretical calculations of the absorption coefficient for direct transitions give the following r w (4o / ncE)(w Eopt. ) 10 2. Theoretical Background A) Indirect Transition: If the bottom of the conduction band Ec occupies the position whose wave vector value of K differs from that for the top of the valence band Ev, then a vertical electronic transition involving only a photon can't directly connect the two electronic states, a photon does not possess enough momentum to ensure conservation of momentum for such transition. Theoretical calculation of the absorption coefficient for indirect transition gives : w ~ [w ( Eg E phon )]2 11 3. Experimental Work 3.1 Material and Sample Preparation: Fig.: Oriented PET samples cut at angle from the IDD (yaxis) sheet IDD IDD IDD 20 40 60 Y IDD = 0 IDD Test specimens were cut from a sheet of draw ration at different angle (0, 20, 40, 60, 75, and 90) from the initial draw direction (IDD at =0) as shown in Fig. The IDD is parallel to the molecular orientation direction. IDD The material used in this work was oriented polyethylene terephthalate (PET) sheet 0.29mm thick and has a density of about 1.3g/cm3, glass transition temperature of about ~69C and melting temperature of about 267C. IDD 75 Tensile specimen 90 X 12 3. Experimental Work 3.2 Annealing Process: This experimental work includes two parts: the first part deals with unannealed (as received) samples of oriented PET, and the second part deals with annealed samples. For annealing test specimens were cut at different angle from the molecular direction (or IDD), and clamped firmly between two smooth plates of steel and after then we are put them in an oven at a temperature of 120C, for 20 hours, then we decreased the temperature slowly until room temperature (~25C). This is the annealing procedure followed in research work. 13 3. Experimental Work 3.3 Optical Measurements: Absorption is expressed in terms of a coefficient (), which is defined as the relative rate of reduction in light intensity. The optical absorbance (A) is taken at wavelength () range (200-800nm) using Cary Photospectrometer in the Chemistry Department of University of Jordan. The absorption coefficient () was calculated from the absorbance (A) Spectra. After correcting the reflection, () was calculated using the relation: I I o exp( ( ) x) Hence ( w) 2.303 I 2.303 log A( ) x Io x Where Io and I are the incident and transmitted intensity respectively and x is the sample thickness. 14 3. Experimental Work 3.4 Estimation of PET Crystallinity: Sample densities were measured using a very sensitive electron balance. Volume fractions of crystallinity were calculated using the following relationship: Xc a c a Where Xc is the volume fraction crystallinity, is the density of the sample, a is the density of 100% amorphous PET, and c is the density of 100% crystalline PET, the values for a (1.335g/cm3) and c (1.455g/cm3) (P.Varma and S.A. Jabarin, 1998). 15 Calculation of crystallinity volume fraction Comparison of A parameters Before Annealing After Annealing Thickness (cm) 0.029 0.031 Length (cm) 5.2 3.4 Width (cm) 1.6 0.08 Volume (cm3) 0.241 0.084 Mass (gm) 0.334 0.117 Density of sample (gm/cm3) 1.386 1.393 Crystallinity (%) 42.5 10 48.4 10 Table shows the difference in the crystallinity (Xc) for oriented PET before and after annealing. The increase in crystallinity of the annealed 16 is about 6% as it was expected for annealing polymers. 4. Results and Discussion In this working, we deal with the effect of annealing on the optical properties of oriented PET samples. This work is done on two types of samples: the first is unannealed (as received) samples and the second annealed samples, with different angle of orientation () of the initial draw direction (IDD) with respect to the y-axis, the optical properties of PET are studied through determination of some physical parameters, such as the optical energy gap,and the energy gap tails. 17 4. Results and Discussion 4.1 Samples Thickness Change: The geometric dimensions of the examined specimens before and after annealing were measured. Table includes the specimens geometry: The geometry of PET specimens with different angles of orientation before and after annealing at 120C As received samples Annealed samples Thickness (mm) Thickness (mm) 0 0.29 0.31 20 0.29 0.31 40 0.29 0.31 60 0.29 0.31 75 0.29 0.31 90 0.29 0.31 Orientation Angle (deg.) The table shows that the thickness of specimens changes with annealing, this was taken into consideration during the results of optical calculations. 18 4. Results and Discussion 4.1 Optical results : The relationship between the fundamental absorption and optical energy gap is given by the relation: Eopt h • c At high absorption coefficient levels, where (w)>104 cm-1, the absorption coefficient for non-crystalline materials has the following frequency dependence (Tauc, 1966, Davis and Mott, 1979). (w)w B(w Eopt )r where B is a factor equals to (4o/ncE), In the case of lower absorption, the absorption coefficient (w) in range (1cm-1 to 104 cm-1), is described by Urbach formula (Urbach, 1953) • (w) o exp( w / E ) 19 Optical result for unannealed oriented PET Optical results for annealed oriented PET Angles of orientation (deg.) Eopt. (eV) B (eV-1.cm-1) E (eV) (Eopt. + E) eV 0 3.770.32 27.83 0.090.01 3.86 20 3.850.4 26.33 0.080.01 3.93 40 3.80.46 27.92 0.090.01 3.89 60 3.760.2 28.23 0.090.01 3.85 75 3.760.25 28.48 0.10.01 3.86 90 3.780.31 29.6 0.10.01 3.88 Angles of orientation (deg.) E opt. (eV) B (eV-1.cm-1) E (eV) (Eopt. + E) eV 0 3.770.11 22.21 0.070.01 3.84 20 3.790.21 22.20 0.070.01 3.86 40 3.790.31 22.94 0.070.01 3.86 60 3.790.41 23.12 0.080.01 3.87 75 3.770.31 23.29 0.090.01 3.86 90 3.740.13 24.52 0.090.01 3.83 20 Optical results of isotropic PET E opt. (eV) B (eV-1.cm-1) E (eV) (Eopt. + E) eV 3.750.2 28.7 0.10.01 3.85 21 4 = 0 oriented PET Absorbance 3 2 1 0 300 400 500 600 700 800 wavelength(nm) Optical absorbance for = 0 of oriented PET before annealing 22 4 Absorbance 3 = 0 oriented PET 2 1 0 300 400 500 600 700 800 wavelength(nm) Optical absorbance for = 0 of oriented PET after annealing 23 3.0 isotropic PET 2.5 Absorbance 2.0 1.5 1.0 0.5 0.0 200 300 400 500 600 700 800 wavelength Optical absorbance for isotropic PET 24 8.0 = 60 oriented PET 7.5 7.0 ln 6.5 6.0 5.5 5.0 4.5 2 3 4 h(eV) Natural logarithm of ( α ) versus the incident photon energy for Ө = 60 of 25 oriented PET before annealing = 60 oriented PET 8.0 7.5 7.0 ln 6.5 6.0 5.5 5.0 4.5 2 3 4 h(eV) Natural logarithm of ( α ) versus the incident photon energy for Ө = 60 26 of oriented PET after annealing 7 ln isotropic PET 6 5 1 2 3 4 5 h(eV) Natural logarithm of ( α ) versus the incident photon energy for isotropic PET 27 10000 = 90 oriented PET 8000 (*h) 1/2 6000 4000 2000 0 -2000 1 2 3 4 h(eV) (αћω)^1/2 versus the incident photon energy for Ө = 90 of oriented PET before annealing 28 10000 = 90 oriented PET 8000 (*h) 1/2 6000 4000 2000 0 -2000 1 2 3 4 h(eV) (αћω)^1/2 versus the incident photon energy for Ө = 90 of oriented PET after annealing 29 10000 isotropic PET 8000 (h) 1/2 6000 4000 2000 0 1 2 3 4 5 6 7 h(eV) (αћω)^1/2 versus the incident photon energy for isotropic PET 30 Y Molecular direction Orientation ellipsoid UV-propagation direction X where is molecular angle rotation with respect to the vertical direction (y-axis) Molecular direction with respect to UV-propagation direction. 31 5. Conclusions 5.1 Conclusions From the analysis of the results obtained, one can conclude the followings: 1. The optical behavior of the given oriented PET polymer depend on angle measured from the IDD. 2. No great changes, before and after annealing, appeared in the observed optical properties Eg and E approximately. 3. The PET samples showed small increase in their thickness through to annealing at 120C, which means that a degree of recrystalization took place in received samples by annealing. 4. The analysis of optical results indicates that the transition energy for electrons is indirect in k-space. 5. The crystallinity was calculated from density measurements. 6. The observed slight changes in the measured optical properties of oriented PET are attributed to some structural changes as crystallinity took place by annealing. 32 33