Ismail Karacan

Characterization of orientation of one-way and two-way drawn isotactic polypropylene films

Abstract X-ray diffraction, Fourier Transform infra-red ( FT i.r.) spectroscopic and refractive index measurements have been utilized to characterize the state of molecular orientation in one-way and two-way (biaxially) drawn isotactic polypropylene (iPP) films. It is shown that the use of all three techniques leads to much greater confidence in the orientation averages deduced than can be obtained by using any two of the techniques. Orientation parameters in terms of 〈cos 2 θ〉 values in machine, transverse and normal directions have been evaluated and compared quantitatively for the different techniques. The results indicate that with one-way drawing the chain axes of both crystalline and amorphous regions orient towards the direction of drawing. The crystalline chains are more highly oriented than the amorphous chains and tend to orient towards the plane of the film, whereas the amorphous chains tend to be more uniaxially oriented towards the draw direction. In balanced, simultaneously two-way drawn films the crystalline chains are more highly oriented towards the plane of the film than the non-crystalline chains. For a sequentially, equibiaxially drawn iPP film, the orientations of the chain axes of both the crystalline and amorphous regions were found to be higher in the second draw (i.e. transverse) direction than in the first draw direction. The orientation of the crystalline chains was very close to the plane of the film, whereas the amorphous chains were almost uniaxially oriented with respect to the second draw direction. In all the films there is a strong tendency for the b-axes of the crystallites to align normal to the plane of the film.

Thermotropic polyesters: structure of oriented poly(p-oxybenzoate-co-p-phenylene isophthalate)s

Abstract Previous work has shown that poly( p -oxybenzoate- co - p -phenylene isophthalate)s are nematogenic within the composition range 33 67 to 75 25 . X-ray investigations of oriented fibres showed that, with respect to the physical structure, they differ from the corresponding unoriented materials. The main emphasis has been placed on structural characterization of fibres from 50 50 and 67 33 copolymers, which have a broad nematic temperature range. The fibres spun from the nematic mesophase were highly oriented but crystalline order was absent. Annealing at temperatures ranging from 240 to 300°C causes crystallization. The structure of annealed fibres can be regarded as a mosaic of thin crystalline regions containing p -phenylene isophthalate sequences, together with paracrystalline regions.

Thermotropic polyesters: modelling of the meridional diffraction patterns of poly(p-oxybenzoate-co-p-phenylene isophthalate) fibres

Previous work has shown that poly(p-oxybenzoate-co-p-phenylene isophthalate) (5050 and 6733) fibres spun from nematic mesophase are highly oriented but lack crystalline order. Heat treatment of these ‘as-made’ fibres results in structure regarded as a mosaic of thin crystalline regions consisting of p-phenylene isophthalate sequences in a paracrystalline matrix. This work is concerned with atomic modelling of the copolymer chains. One-dimensional Fourier transforms generated from the models are compared with the observed meridional diffraction traces of the ‘as-made’ fibres. A good match is obtained for the extended chain models containing ‘trans’ conformations of p-oxybenzoate and p-phenylene dioxy units.

Molecular orientation in uniaxial, one-way and two-way drawn poly(vinyl chloride) films

Abstract Uniaxially oriented poly(vinyl chloride) films drawn at 90, 100 and 110°C to draw ratios in the range 1.5–3.5 have been investigated using i.r. spectroscopy and refractive index measurements. Their 10 s isochronal extensional creep moduli and dynamic tensile moduli have also been measured. One-way drawn films with draw ratios in the range 1.56 × 1–2.56 × 1 and simultaneously and sequentially two-way drawn films with draw ratios in the range 1.46 × 1.46–1.79 × 1.79 have been investigated using refractive index measurements and i.r. spectroscopy. These films were drawn at 85 and 95°C. Thermal analysis of the drawn samples showed that the uniaxially drawn films were drawn at temperatures well above their final glass transition temperatures, whereas the one-way and two-way drawn films were drawn close to their glass transition temperatures. The results show that the crystallites are more highly oriented than the amorphous chains and that one-way drawn samples depart slightly from uniaxiality. No significant difference in orientation was found between simultaneously and sequentially two-way drawn samples. It was shown that the i.r. peak at 613 cm −1 , attributed to short syndiotactic segments, could be used to characterize the orientation of the amorphous chains. The elastic moduli of the uniaxially drawn samples were found to depend on orientation in a similar way to that previously reported.

The Influence of Thermal Stabilization Stage on the Molecular Structure of Polyacrylonitrile Fibers Prior to the Carbonization Stage

Structural transformation from a linear chain structure to crosslinked chain structure, occuring during the thermal stabilization stage of carbon fiber manufacture, was followed through the use of infrared spectroscopy, differential scanning calorimetry (DSC), elemental analysis and gel-fraction measurements. The results obtained from the analysis of IR spectroscopy showed the gradual and continuous loss of intensity of the nitrile (C≡N) vibration at 2242 cm−1 closely associated with the cyclization reactions whereas the intensity loss of the methylene (CH2) groups vibration around 2920–2939 cm−1 has been attributed to the loss of hydrogen atoms as part of the dehydrogenation reactions. The dehydrogenation index, evaluated using the absorbance ratio of A1452/A1368, also indicated the gradual loss of hydrogens in agreement with decreasing hydrogen content with progressing stabilization process. IR spectroscopy also showed the emergence of new absorption bands attributed to the formation of crosslinked ladder-like structure in the 1590–1600 cm−1 region. The amount of newly formed crosslinked structure was characterized using DSC conversion index, IR conversion index and gel-fraction measurements. The results are compared and discussed in detail.

Thermotropic polyesters : orientation of poly(p-oxybenzoate-co-p-phenylene isophthalate) fibres

Abstract Orientation of poly(p-oxybenzoate-co-p-phenylene isophthalate) ( 50 50 and 67 33 ) fibres spun from the nematic mesophase is investigated by wide-angle X-ray scattering. Computer-generated sets of model chains and their two-dimensional Fourier transforms are used to interpret the experimental results. It is found that the orientation parameters evaluated from the azimuthal spread of the diffuse equatorial scatter, assigned to the paracrystalline structure in the ‘as-made’ and heat-treated fibres, underestimate significantly the alignment of chains along the fibre axis.

The role of thermal stabilization on the structure and mechanical properties of polyacrylonitrile precursor fibers

The thermal stabilization stage of polyacrylonitrile (PAN) fibers is characterized by a steady and continuous reduction in fiber diameter and linear density values together with color changes from reddish brown to shiny black with increasing stabilization time. Thermally stabilized PAN fibers acquire infusible and nonburning characteristics prior to the carbonization stage. Structural characterization of thermally stabilized polyacrylonitrile fibers was carried out using an indepth analysis of equatorial X-ray diffraction traces. Curve fitting of X-ray diffraction traces provided accurate peak parameters which were subsequently used for the evaluation of apparent crystallinity, apparent crystallite size and X-ray stabilization index. The results showed the loss of crystallinity due to the amorphization processes together with a steady and continuous decrease in lateral crystallite size with increasing stabilization time. With the progress of thermal stabilization, a new amorphous phase with a crosslinked and aromatized structure is formed which is expected to withstand high carbonization temperatures. Mechanical properties of the thermally stabilized PAN precursor fibers were found to be adversely affected with the progress of stabilization time. Due to the influence of thermal degradation mechanisms heavily involving chain scission along the fiber axis direction, tensile strength and tensile modulus values were found to decrease by significant proportions with the prolonged stabilization times.

An in depth study of crystallinity, crystallite size and orientation measurements of a selection of poly(ethylene terephthalate)fibers

A selection of commercially available poly(ethylene terephthalate) fibers with different degrees of molecular alignment and crystallinity have been investigated utilizing a wide range of techniques including optical microscopy, infrared spectroscopy together with thermal and wide-angle X-ray diffraction techniques. Annealing experiments showed increased molecular alignment and crystallinity as shown by the increased values of birefringence and melting enthalpies. Crystallinity values determined from thermal analysis, density, unpolarized infrared spectroscopy and X-ray diffraction are compared and discussed in terms of the inherent capabilities and limitations of each measurement technique. The birefringence and refractive index values obtained from optical microscopy are found to decrease with increasing wavelength of light used in the experiments. The wide-angle X-ray diffraction analysis shows that the samples with relatively low orientation possess oriented non-crystalline array of chains whereas those with high molecular orientation possess well defined and oriented crystalline array of chains along the fiber axis direction. X-ray analysis showed increasing crystallite size trend with increasing molecular orientation. SEM images showed micro-cracks on low oriented fiber surfaces becoming smooth on highly oriented fiber surfaces. Excellent bending characteristics were observed with knotted fibers implying relatively easy fabric formation.

A study on structural characterization of thermal stabilization stage of polyacrylonitrile fibers prior to carbonization

Structural transformations taking place during the thermal stabilization of polyacrylonitrile (PAN) fiber used for the production of carbon fiber were characterized using a combination of polarized infrared spectroscopy, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and density measurements. Direct relationship between the increasing oxygen content and the density values was confirmed with increasing stabilization time. Linear density values were found to be directly influenced by the stabilization time. Thermal stability of stabilized precursor fibers was evaluated in terms of weight loss and residual weight fraction. The results showed that a residual weight fraction of 65 % at 1000 °C can be obtained but longer stabilization time resulted in a loss of residual weight fraction due to excessive thermal degradation. SEM was used for the observation of surface morphological features of stabilized precursor fibers. Polarized infrared spectroscopy showed the loss of molecular orientation of methylene (CH2), nitrile (Ct=N), and carbonyl (C=O) groups in direct response to the effects of cyclization, dehydrogenation, and amorphization (i.e. decrystallization) processes taking place during the stabilization stage.

Thermotropic polyesters : thermal transitions and polymorphism of poly(4-oxybenzoate-co-1,4-phenylene isophthalate)s

Abstract Differential scanning calorimetry and wide-angle X-ray diffraction techniques are employed to investigate the melting behaviour and crystalline structure of poly(1,4-phenylene isophthalate) and poly(4-oxybenzoate- co -1,4-phenylene isophthalate) containing equimolar quantities of the constituent 4-oxybenzoyl, 1,4-phenylenedioxy and isophthaloyl units. These materials can contain two different crystalline structures, both consisting of 1,4-phenylene isophthalate sequences. The formation of these polymorphs in both the homopolymer and copolymer is influenced by the crystallization and/or annealing conditions. The melting behaviour of the copolymer is strongly influenced by annealing, but these changes can be reversed by remelting. Consequently, they are not caused by changes in chain constitution arising from transesterification.