The performances of the non-isotropic polymer systems strongly depend on their super molecular structure (Wu et al., 2001; Shabana, 2004; Keum & Song, 2005; Ziabicki & Jarecki, 2007; Sulong et al., 2011).
The wide application and consequently higher production of flexible chain fiber forming polymers, in particular poly (ethylene terephthalate) (PET) is due to the possibility of the heat mechanical modification to obtain highly modular and high strength materials from them (Llana & Boyce, 1999; Bai et al., 2000; Dupaix & Boyce, 2005; Guzzato et al., 2009). PET is an essential engineering polymer with properties strongly depending of the degree of crystallinity and the perfection of crystal phase, too. The effects of some basic parameters of the heat mechanically treatment such as strain force extension rate and temperature on the structure development of PET have been studied using different methods as differential scanning calorimetry (DSC), wide angle X-ray scattering (WAXS) (Kong & Hay, 2003; Zhang et al., 2004; Karagiannidis et al., 2008; Raabe et al. 2004), dynamic mechanical analysis (Ma et al., 2003), laser irradiation (Wijayathunga et al., 2007) and other.
The optimal performance of the high-temperature orientation modification is a complicated and still not sufficiently well studied process. The simultaneous mechanical and thermal modification however is extremely complex phenomenon occurring on the basis of statistical probabilistic processes, as are also the possible results from it. In this sense the results from variations of heat mechanical modification are unpredictable not unique and often very different, contradictory and unexpected. Moreover for each specific object and purpose exist additional conditions, and therefore needed special study of orientation thermal treatment for the obtaining of best mechanical performance. If the samples simultaneous heat mechanical modification (SHMM) is carried out without accounting and control of a number of events, processes and parameters the results can easily prove contrary to the expectations. And to make the results from STMM easily predictable, susceptible to control and allowing obtaining of materials with improved predefined wanted properties it is necessary in depth study on the nature, mechanism and kinetics of the justifying processes and the relationship between them. Therefore the study of these processes is a permanent "ever green" interest in the polymer physics. One of the most interesting from this point of view objects are polyethylene terephthalate fibers. There are varieties of investigations of the affects of the thermal and mechanical treatments on the relaxation and phase transitions in PET fibers. In some of them as-spun filaments are subjected to thermal treatment at constant temperatures without stress (Betchev, 1995; Bai et al., 2000) as well with application of tensile force (Zhang et al., 2004; Sharma et al., 1997).
Important is the answer of the question on what schemes and under what conditions should be conducted SHMM to maximize the orientation effect at the expense of minimal object destruction at high temperature uni-axial deformation. To obtain definite answer to a similar question is necessary a multifactorial planning and carrying out of massive diverse experiment. The preliminary suggestive for a range of the possible conditions of withdrawal experiments are impressive much.
For initial approbation of the behavior of the specific object to the complex SHMM we accepted the technologically real (and maximum possible) temperature interval from 20º º C to 200º º C and sufficient as a beginning, a range of orientation tensions from 0 MPa to 1.7 MPa with enough good resolution of 0.1 MPa. The experiment was carried out in combination of gravitational loading of the samples at a linear heating in line with the coefficient of fibers thermal conductivity average heating rate of 3.5º º C/min.
The dependence of the relative deformation from the tensile load values showed an initial intensive growth of the gradient of its increase up to strain stress value of 0.7 MPa, probably because of intensive destruction of macromolecular segments in the studied samples. A similar information was emitted and from the other performed structural analyses. The results led us to include new elements into the idea of the experiment and in particular to eliminate the adverse action of destructive tensions above those causing bundle deformation 290 %. Results showed that above loading of 1.2 MPa the relative samples elongation falls below the above mentioned value of the bundle reletive elongation and is no need to limit it. In the new version the thermal deformation experiment was carried out without limitation of the bundle extension at combination of the samples gravitational loading in the range from 0 MPa to 3.0 MPa with a good resolution of 0.12 MPa at a linear heating with the same heating rate (3.5º º C/min) and again in the temperature range from 20º º C to 200º º C. The structural tests of the SHMM samples in this preliminary experiment showed the disadvantages of the wide temperature range. Therefore, were tested modifications of PET fibers at well defined temperatures of 80º º C, 85º º C, 90º º C and 95º º C in the temperature range just above the glass transition temperature of the objects defined in our other investigations of 74º º C. The samples were loaded gravitationally (with different orientation tensions with initial values of 40 MPa, 80 MPa and 120 MPa, varying during the deformation downloading) as well as with constant rate of loading 0.1 m /min up to various relative elongations of 20 %, 40 % and 60 %.
PET undrawn multifilament yarns produced by melt spinning on the industrial spinning installation Furnet (France) have been selected as a precursor samples. The technological parameters and basic characteristics of the original filaments are shown in Table 1.
It can be seen from the Table 1 that within the group of the selected samples have both amorphous and partially crystalline filaments. The selected specimens are spun at different spinning speeds and thus with different preliminary orientation. So they are suitable for the achievement of the above-defined purpose of the present study.
2.2.1. Simultaneous heat-mechanically modification (SHMM)
Different versions of simultaneous thermal and mechanical treatments of the studied yarns were performed using devices constructed and produced in our laboratory.
The first version of SHMM includes linear samples heating from room temperature up to 200º º C accompanied by applied to the fiber bundle strain stress. The heating rate was 3.5º º C/min. The used gear consists of a vertically located cylindrical furnace, which moves around a rolled up PET bundle fixed by special holders and subjected to needed tensile stress. The temperature reaching of 200º º C was followed by a simultaneous termination of the tensile stress and the yarn remove from the oven at room temperature. Highly supercooled i.e. deep tempered and isothermally crystallized at temperatures close to the melting temperature thin films PET, used for forming of the investigated fibers are shown in Fig. 1a, b, c and d respectively.
In the second variant of SHMM the investigated filaments were subjected to tensile stresses with different values under certain constant temperatures. The simultaneous heat mechanical samples modification was carried out using an apparatus created in our laboratory. The device involves a movable cylindrical oven located on the horizontal rails and a setup for the sample deformation reading. The heat-mechanical treatment begins when the preheated oven was rapidly shifted around the studied PET bundle that was simultaneously stretched with the needed strain stress. The experiment involves annealing of an as-spun PET yarns at four different temperatures in a narrow temperature range from 80º º C to 95º º C closely above its glass transition temperature while they are subjected to a well-defined tensile stress.
In the next version of SHMM the studied PET filaments were subjected to extension at a constant speed and constant temperatures in the same temperature range from 80º º C to 95º º C. The structural characterizations of the studied fibers after the above described heat-mechanical treatments were realized using differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS) measurements.
2.2.2. Differential scanning calorimetry (DSC)
Part from the calorimetric studies was performed on a Mettler-Toledo heat-flux calorimeter DSC 820 with liquid nitrogen accessory. The furnace was purged with nitrogen at a flow rate of 80 ml/min. Temperature calibration was done using the onset melting temperatures of indium and zinc, and the energy calibration was based on the heat of fusion of indium. Fibers were cut in pieces of less than 1 mm and sealed in standard 40 μl aluminum pans.
Another part of the calorimetric analysis was carried out using a NETZSCH heat-flux calorimeter STA 449 F3 Jupiter (TG/DSC) in static air atmosphere. Temperature calibration was done using the onset melting temperatures of indium, tin, bismuth and zinc, and the energy calibration was based on the heat of fusion of the same metals. Fibers were cut in pieces of less than 1 mm and sealed in standard 85 μl platinum pans.
2.2.3. Wide-angle X-ray scattering (WAXS)
The fiber structure was studied by wide-angle X-ray scattering (WAXS), too using two different apparatus namely:
Diffractometer HZG 4 (Freiberger Präzisionsmechanik, Germany) and Ni-filtered Cu radiation with wavelength λ = 1.5418 Å. Equatorial scattering was monitored in transmission mode. The fiber samples were prepared as a layer with 2 mm thickness and 10 mm width, and mounted on the sample holder of the diffractometer;
Diffractometer URD - 6 (under license of SIEMES) of the company "Freiberger Präzisionsmechanik" (Freiburg im Breisgau, Baden-Württemberg, Germany). Used is β-filtered with Ni-filter Cu radiation with a wavelength λ = 1.5418 Å.
3. Results and discuss
3.1. Investigation of amorphous PET fibers simultaneous heat - mechanically modified at linear heating and constant strain stress values
The study of the relationships between the SHMM modes and subsequent structural development in the PET filaments includes different versions of experiments.
In the first one amorphous fibers marked as sample A (Table 1) were subjected to SHMM at conditions as follows: Heating with linear increasing of the temperature in a wide range from 20º º С to 200º º С with heating rate of 3.5º º С/min under constant strain stress from 0 МРа to 1.7 МРа (increasing step of 0.1 МРа). It should be noted the additional experimantal conditions for some of the samples. The extension of the yarns loaded with tensile stress from 0.7 МРа to 1.2 МРа was limited up to 290 %. Moreover after the reaching of the limited bundle length the sample continues to be heated up to 200º º C.
The length changes of the investigated yarns were registered during their combined heat mechanical treatment. As expected the filaments retain initial dimensions in the temperature range from room temperature up to 75º º C. In this temperature interval samples remain in glassy state and the structural changes are negligible.
The changes of the bundle dimensions depend on the applied strain stress level considerably and strongly at temperatures between 80º º C and 130º º C. The observed dependence can be explained with the sample transition from glassy to rubbery state. The deformation behaviour demonstrated by the samples at a level of applied tensile stress up to 0.7 MPa is expectable. Experiments showed a decrease of the final bundle length at small stress values. The filaments shrinkage can be logically explained with the process of frozen internal stresses relaxation in the samples at the temperature range of the transition from glassy to rubbery state.
Unexpected and quite interesting was the deformation behavior of the bundles subjected to stresses in the range from 0.7 MPa to 1.2 MPa. As it was mentioned above the extension of the samples tested with tensile stress from 0.7 МРа to 1.2 МРа was limited up to 290 %. The observed deformation behavior strongly corresponds to the so-called fluid-like deformation. The bundle length was kept constant when the extension reached 290 %.
It is important to underline that such an intensive fluid-like deformation process was not observed for the samples subjected to strain stresses above 1.2 MPa. The received experimental data showed a decrease of the elongation with the stress values increasing.
Structural changes in the PET fibers as a consequence of the simultaneous thermal and mechanical treatments were studied using DSC, wide angle X-ray scattering (WAXS) and density measurements. The changes of the samples degree of crystallinity estimated on the basis of the DSC data depending of the strain stress values are presented in Figure 2. As it can be seen from Figure 2, some of the studied specimens are semi-crystalline while others are practically amorphous. The comparison with the SHMM conditions shows that the samples subjected to tensile stresses in the intervals from 0 MPa to 0.6 MPa and from 1.3 MPa to 1.7 MPa posses semi-crystalline structure. At the same time, the filaments with limited ability for extension treated in the stress interval from 0.7 MPa to 1.2 MPa are practically amorphous. Density measurements and WAXS diagrams proved the same crystallization properties of the studied PET specimens, too.
Representative DSC thermograms of partially crystalline and amorphous PET fibers subjected on heat mechanically treatments under the above decriebed conditions are present on Fig. 3, 5 and 4 respectively. As expected the DSC curve of the raw amorphous sample show pronounced cold crystallization and melting peaks. Unlike the untreated fibers, DSC thermograms in Fig. 3 and Fig. 5 show only preliminary melting and melting endotherms without cold crystallization peaks. Moreover the peak temperature of the premelting and melting endotherms in Fig. 3 smoothly shifts to higher temperatures with stress increasing. Multiple melting peaks in PET pellets (Kong & Hay, 2003) and filaments are observed and studied in earlier investigations. Similar to the raw PET filaments the DSC curves presented on Fig. 5, of the bundles subjected to SHMM at limited extension show glass – rubber transition, cold crystallization and melting peaks. Also it can be seen from Fig. 5 that the tensile stress increasing leads to fluently displacement of the cold crystallization peak to lower temperatures and to sliding to higher temperatures of the melting peak.
In conclusion it can be said that the heating with linear temperature rise, accompanied by application of external strain stresses strongly influences the nature of structural rearrangements in the investigated uncrystallized PET filaments. The observed fibers net deformation at tensile stress values less than 0.7 MPa and more than 1.2 MPa can be explained with a faster crystallization of the amorphous PET bundle from rubbery state, as a consequence of the influence of the applied tensile stress. The fluid-like deformation process predominates when the applied stresses are from 0.7 MPa to 1.2 MPa. It was found that after heating up to 200º º C amorphous PET filaments could preserve the amorphous state when the applied external strain stresses are in the same range.
At the same time questions having fundamental and practical aspects remain without clear answer and namely: What is the role of the restrictions and mechanical stress in obtaining of such qualitative different results? What would be the bundle deformation behaviour if there were no restrictions? What is the influence of the regime of heat treatment?
With purpose to clarify the role of the applied strain stress on the fibers structure development it was interesting to realize the above-described experiment without the mentioned limitations.
|№||σ, MPa||№||σ, MPa|
In the next thermal deformation experiment amorphous PET fibers named sample B (Table 1), were linearly heated with rate of 3.5º º С/min from 20º º С to 200º º С. During the filaments heating they were subjected to constant tensile stress in a wider range from 0 МРа to 3.0 МРа (increasing step of 0.12 МРа, Table 2) without restrictions of the bundle deformation. The bundle length obtained after the heat mechanical treatment as a function of the applied strain stress is presented in Figure 6, where the dashed line marks the initial sample length.
The deformation behaviour demonstrated by the samples at a level of applied stress up to 1.68 MPa is expectable. Experiments showed entropy shrinkage of the first four samples at small stress values up to the level of 0.36 MPa. The filaments shrinkage is a consequence of the frozen internal stresses relaxation at the temperature range of the sample glass transition. It could be supposed that the applied (external) stresses in our experiments up to value of 0.36 MPa do not compensate the emerging shrinkage forces.
Significant sample extension is stimulated by the stress increasing from 0.36 MPa up to 1.68 MPa. As it can be seen from the results presented in Figure 6 only increment of the final bundle length can be observed in this case. Obviously such of dependence can be detected when the applied strain stress is higher than the potential entropy shrinkage forces in an amorphous uniaxially oriented sample within the temperature range of glass transition. The received experimental data strongly corresponds to the so-called fluid-like deformation. At stress value of 1.68 MPa is reached more than fivefold bundle monotonic download. This is the maximum achievable prolongation by used method and conditions of SHMM.
Much more interesting and non-expectable is the deformation behaviour of the samples subjected to stresses in the range from 1.8 MPa up to 3.0 MPa. As it is illustrated on the Figure 6 the increasing of the tensile stress values from 1.68 MPa to 2.16 MPa leads to gradually decrement of the final bundle length. A significant reduction of the net deformation occurs at the stress levels of 2.28 MPa and more. Despite of the rise of the applied stress values the samples extension decreases considerably. Moreover the change of the tensile stress does not affect the deformation behaviour of the last seven yarns. Their ultimate length is more than twice less than the maximum achieved nder stress value of 1.68 MPa. Depending on the deformation behavior the investigated samples can be conditionally divided into three groups as follws. First one includes the bundles with numbers from one to fifteen. In the second one are the yarns from sixteen to twenty, and the third group includes the last six specimens which despite of the stress values increasing are extended less. Structural rearrangements occurred in the PET fibers as a result of the SHMM were studied using DSC and WAXS. It should be underlined that in contrast to the previous experiment the performed structural analysis show that all of the heat mechanically modified PET filaments are partially crystalline. Representative DSC curves of melting peaks of the above defined three groups of samples are present in Figures 7, 9 and 11. As it is visible from Figure 7 during the SHMM are formed three types of structures with three different types of perfection and stability. Depending on the melting temperature can be distinguished entities with a higher level of order forming an easy fusible mesophase, middle crystalline phase with lower perfection and main crystalline phase. Samples heating whether without load, forms easy fusible structure, which melts at about 190º º С. Just small increasing of the stress values leads to the structure improvement and stabilization and to the moving of the mesophase melting temperature to higher temperatures up to around 210º º С - 215º º С. The intermediate crystalline phase with lower perfection is observed as splitting of the main melting peak which visible migrate to the higher temperatures. With the tensile stress increasing the first melting peak as well as the main melting peaks are deformed with a tendency to split. The melting peaks also fluctuate around an average melting temperature significantly higher in comparison with the obtained without load. The observed shifting of the endo effects at higher temperatures possibly is a consequence of more organized structure formation due to the applied orientated pulling load. Only the sample from this group loaded with stress of 1.68 MPa show a slightly different melting behavior.
Representative wide-angle X-ray scattering curves of SHMM PET fibers from the above defined three groups of samples are present in Figures 8, 10 and 12. The diffraction curves are presented to illustrate the change in the fibers degree of crystallinity and orientation with the samples load increasing as well as their compliance with the DSC curves of the same objects for comparison of the structural information from the both methods wich are respectively geometric and energetically-structural sensitive. The first group of samples is characterized by a monotonic, although nonlinear elongation increase with the strain stress increasing up to the specimen with number 15 (Fig. 6). As is evident from Fig. 8, with the load increasing within this group the intensity distribution in the diffraction pattern shows noticeable changes with the stress increase, which is evidence for the significant structural reorganization without strict consistent trend observed in a specific type of amendment.
Representative DSC curves of melting peaks from the second group of samples are given in Figure 9, where is observed a tendency on clearing of any thermo-effects with increasing of the strain stress values. That could be explained reasonably from viewpoint of the model for the homogenization of the structure at medium load values and respectively maximum deformation (Fig. 6). The samples deformation behavior corresponds to heat mechanicaly modified fibers stretched up to position in which the carrying are the most widespread fractions in the amorphous regions according to the normal Gaussian distribution. It is logically that the orientation processes at this group to be most effective with predominance of the orientation processes over destructive.
In the second group of WAXS curves (Fig. 10) occurred more stable trend of increase in the intensity of the difraction radiation from improved crystalline and oriented regions in the samples. The DSC curves from the same group (Fig. 9) also showed a stable trend of deviation of the melting process to higher temperatures which confirms the suggestion for improvement of the crystalline phase. The same is valid and for the oriented amorphous regions. As from the diffraction and DSC curves, as well as from the stress - deformation dependence is confirmed the assumption for additional objects orientation allowing improvement of the crystalline phase and the supporting fraction in amorphous sections, which leads up to decrease in the total relative fibers deformation.
The DSC thermograms of the samples from the third group are present in Figure 11. It can be seen that with the tensile stress increasing endo peaks shift to lower temperatures. And it is valid for both before melting and the main melting processes. The most likely reason for the observed effect is that this third group of samples was withdrawn most suboptimal, with a predominance of destructive processes over the orientation. As a result, the obtained structure is mechanically and thermodynamically unstable, with the lowest density and perfection and therefore melts most easily at lower temperatures. As in Figure 9 as well as in Figure 11 are seen beginnings of split of the main melting peaks. The reasons for the splitting of the melting peaks may be different. In this case, at this type of heat mechanically fibers modification, the splitting occurs most probably due to structural reorganization during the melting process.
With the strain stress increasing at the samples with numbers from 20 up to 26 in which the relative fibers deformation almost does not change (Fig. 6) the intensity of the diffraction reflections significantly increase (Fig. 12). Probably for the account of low elastic deformation is realized a significant improvement of the crystalline phase on the surface of the lamellae or in the newly oriented regions. The exception occurs only in the sample subjected of the biggest tension stress of 3.0 MPa, where the intensity of the diffraction pattern falls strongly. Perhaps the increased destruction of separate fractions of macromolecular segments partially distorts the degree of the orderliness in the polymer system. At the same time the indestructable part of additional downloaded segments further improves their arrangement so that is realized more detailed infrastructure of distribution of the intensity of the powder diffraction pattern. The thermal curves (Fig. 11) show logically enhancement of the high temperature melting fractions with the tension stress increasing, as well their visible split.
On the basis on the carried out structural investigations of amorphous PET fibers simultaneous heat - mechanically modified at isothermal conditions and constant strain stress values it can be make the following conclusions: The mechanical strain force applied simultaneously with the linear heating of the studied PET yarns affects significantly the deformation behaviour and samples crystallization kinetics. Moreover in contrast to the results obtained in the first experiment, all of the so treated specimens are partially crystalline. The role of the tensile stress in the adjustment of the interacting processes of the fluid like deformation and stress-induced crystallization clearly reveals in the ultimate samples deformation. At stress values from 1.56 MPa to 2.16 MPa predominates the fluid like fibers extension, while the further stress increasing leads to the earlier crystallization start and thereby to decrease of the final fibers length.
3.2. Investigation of amorphous PET fibers simultaneous heat - mechanically modified at isothermal conditions and constant strain stress values
The glass transition temperature of the amorphous sample B (Table 1) was determined of 74º º C in our previous work. It was very interesting to follow the influence of SHMM on the structural development of studied samples in the transition temperature range between the glassy and rubbery fibers state.
With purpose to clarify the role of the temperature and tension stress values on the structure development in amorphous PET yarns, were carried out thermal deformation experiments at constant temperatures in a narrow temperature range from 80º º C to 95º º C. The experiment involves annealing of an as-spun PET bundles at temperatures of 80º º С, 85º º С, 90º º С and 95º º С accompanied by precisely defined tensile stresses from 0 MPa to 30 MPa with increment step of 3 MPa. The samples were loaded during two minutes with tensile stress after ten minutes annealing.
The yarn length obtained after the above described heat mechanical treatment as a function of the applied strain stress is presented in Figure 13 where the dashed line shows the initial sample length. As is apparent from Table 1 the sample B is almost completely amorphous, possess relatively low birefringence value and therefore weak pre-orientation of the amorphous phase. Specimen B is subjected to SHMM with a good resolution in the particularly effective temperature range above the glass transition. The curve obtained at temperature of 80º º С shows the initial level of the material deformation at selected conditions. The bundle deformation behavior (at 80º º С ) is probably due to a slight stretch, uncoupling and disentangle of the free and slightly folded segments in the amorphous areas. Similar effects are observed up to stress value of 9 MPa, but mainly at stress levels between 3 MPa and 6 MPa. At loading above 9 MPa the curve comes to the a plateau and up to normal load of 30 MPa the structure is stable.
The temperature raising to 85º º С and 90º º С shows a common way of the obtained curves where can be seen an initially entropy contraction for the unladen samples and a significant increasing of the relative residual deformation at stress level of 3 MPa. After the initial sharply increase of the ultimate bundle length both curves indicate only small increments of the deformation and orientation with load increasing. Behavior of the curve at 95 ºº С displays much higher values of the sample deformation, which growing with the load values rising. The observed trend can be result from reinforced destruction and slip the segments instead of unfolding and orientation, i.e. from sub-optimal fibers straining. The differences in the bundles deformation behavior, depending on the load values and temperature show that the selected interval of the complex stress - temperature conditions is a suitable to realize the objectives of the thermal deformation experiment.
The structural analyses of the amorphous PET yarns heat-mechanically treated at the above-described conditions were carried out using differential scanning calorimetry and birefringence measurements. The clearly expressed cold crystallization (Fig. 14) of the samples modified at 80º º C is due to the presence of disordered and unstable structure in the unorientated amorphous fibers. In such structures, heated up to temperatures around 130º -140º º С in the absence of external stress (0 MPa) is released the segment mobility in macromolecules, which is a precondition for the cold crystallization. For similar structures the applied stress of 3 MPa at temperature of 80º º C is not enough to cause noticeable fibers orientation and structure stabilization. The melting peaks of these samples are broader and imperfect because the melting polymer system is imperfect, contains less and more defective crystalline phase.
With increasing of the tensile stress values from 6 MPa and more (Fig. 15) the cold crystallization peak at about 140º º С gradually disappears but appears a new similar peak at much lower temperatures, immediately after the transition from glassy to rubbery state. Probably the higher stress further destabilize the structure and facilitates the glass transition proces. Furthermore it is observed an increase of the melting enthalpy and monotonous proportional dependence of the melting temperature increasing with the load growth and respectively the quality and quantity of the crystalline phase. And more essential, in accordance with the crystalline phase and structure improvement the low temperature cold crystallization moves to the lower temperatures. Consequently the more improved and arranged structure crystallizes at lower temperatures in the amorphous phase, while more difficult at higher temperatures melts the more perfect crystal phase.
Except the study by differential scanning calorimetry was measured and the birefringence as a function on the stress of the heat – mechanically treated at 80º º C fibers. It is interesting to note the appearance of ”resonance” filaments birefringence with a pronounced maximum at tension stress value of 6 MPa which is an additional evidence for the role of the applied stress in the filaments structure rearrangement. With increasing of the SHMM temperature from 80º º C to 85º º C (Fig. 16) and under minimum load of 3 MPa losing the effect of the samples cold crystallization at temperatures around 130º - 140º º С. Instead appears the above described cold crystallization at lower temperatures around 110º º C. It is observed an increasing of the melting temperature and enthalpy as well as the perfection of the melting crystalline phase. At middle and higher loads a cold crystallization at 130º - 140º º С does not happen. It is observed the same effect of lowering of the temperature and intensity of the earlier cold crystallization. With the loads increasing is enhanced the tendency for splitting of the melting peaks as well as homogeneous proportional increases of the melting temperature. An interesting fact is that at load of 6 MPa has spilled, extended over a wide temperature range endo effect of enhanced segmental mobility and entropy and at stress of 12 MPa has rudiment of one.
With purpose to specify the occurred in the samples structure developments caused by the SHMM at temperature 85º º C has been measured and the fibers birefringence. The obtained results pertaining to the birefringence values depending of the applied load show a wide basic peak at stresses of 12 MPa, 15 MPa and 18 MPa with two attendant smaller peaks at stresses of 3 MPa and 27 MPa correspondingly.
Very interesting is the DSC curve obtained without loading (strain stress value of 0 MPa) and temperature of 90º º C (Fig. 18). It is impressive much larger peak of cold crystallization in comparison with all other temperatures. This is probably the most suitable temperature for heat mechanical modification of the studied amorphous PET fibers, with the relevant background. At this temperature are released large resources for structural reorganization. From the same graph (Fig. 18) is seen that the effect of restoration of the segmental motion, known also as alpha-relaxation transition is practically suppressed. Perhaps during of the SHMM directly achieves some effect of low-temperature arrangement in the free state of the object at load absence. The fact that only at this temperature takes place so intensive cold crystallization, with pronounced additional crystallization (or pre-crystallization), just before the samples melting is evidence for the optimal combination of the initial structure of the object with the selected temperature of SHMM. As at temperatures of 80º º C and 85º º C (Fig. 15 and 17) as well at 90º º C is observed a complex nature of the cold crystallization again under load of 6 MPa.
As is evident from Fig. 20 at temperature 95º º C the amorphous samples heat mechanically processed at lower loading values indicate a decreasing intensity of cold crystallization and
rising thermograms inclination. Possible reason for the observed tendencies are due to the overall increase of the entropy of the system with the samples heating, resulting in the amend of the thermal conductivity and heat capacity of the objects. In the field of the medium and high loads (Fig. 21) appears the same effect of the lowering of the temperature and intensity of the cold crystallization and homogeneous proportional increasing of the melting temperature. With the loads increasing are monitored and enhanced split of the melting peaksр too. It is reasonable that with the stress values rising to differentiate two phases in the studied samples with structural differences between them and accordingly with different melting temperatures.
On the basis of the data obtained by DSC was calculated and the degree of crystallinity of the studied PET fibers. The obtained results are presented in Table 3. If compare the data given in Table 3 with the relative change of the bundle length (Fig.22) will be seen certain similarity in the change the samples degree of crystallinity and fibers deformation behavior. The bundle deformation behavior observed at temperature of 80º º С (Fig. 22) shows a relatively large yarns extension with the stress level increasing from 3 MPa to 6 MPa. As it is visible from the Table 3 the samples degree of crystallinity at the same temperature (80º º С) and load levels (3 - 6 MPa) grows with more than thirty percents. Further more, at loadings above 9 MPa both the final bundle length and filaments crystallinity comes to a plateau up to load value of 30 MPa.
Very interesting are the changes of the filaments degree of crystallinity obtained with the temperature rising to 85º º С and 90º º С where a sharp increase of the samples crystallinity occurs even at stress value of 3 MPa. Like the bundle deformation behavior (Fig. 22) after the initial sharply increase of the filamemts crystallinity, both dependencies practically follow a common course. At loadings above 6 MPa they are coming to a plateau up to stress values of 30 MPa. Similar is the course and of the fibers degree of crystallinity at temperature 95º º С. It can be concluded that the samples degree of crystallinity reasonably good follows the bundle deformation, which is additional proof for the role of the strain stress in the crystallization of the studied PET fibers.
3.3. Investigation of partially crystalline PET fibers simultaneous heat - mechanically modified at isothermal conditions and constant strain stress values
The next stage from the the realized studies was to investigate the role of the SHMM conditions on the structure developments of partially crystalline PET filaments with different preliminary orientation.
With that purpose the samples named S1, S2 and S3 (Table 1) were heat – mechanically treated at constant temperatures in the same transient range from glassy to rubbery state. The thermal deformation experiment involves bundles annealing at temperatures of 80º ºС, 85º ºС, 90º ºС and 95º ºС accompanied by constant gravimetric tensile loadings of 40 MPa, 80 MPa and 120 MPa during two minutes. In comparison with the above described experiments the applied strain stress values are significantly larger. The experimental conditions allow to trace the combined unfluence of the tensile stress, temperature and preliminary fibers orientation on the going in the samples structural changes.
The structural analyses of the PET filaments subjected to SHMM at the above-described conditions were carried out using differential scanning calorimetry and birefringence measurements.
Representative panels of DSC curves of sample S1 obtained at temperatures 80º º C and 95º º C are present in Figures 22 and 23. The DSC curves of the heat mechanicaly treated samples show very important differences due to the structural reorganization occurred as a result of the modification. There is a radical change of the objects behavior in the temperature regions of the segmental motions defrosting (glass transition) and an increase in the macromolecules mobility (melting). In the areas of the hardening (change of the thermodynamic, entropy mobility of the polymers) and liquefying (increased viscous-liquid mobility) are observed visible structural changes with a steady trend with increasing the load and temperature. Probably due to the regulating effect of the SHMM on the structural organization the glass transition disappears together with the cold crystallization relatively monotonous. After samples SHMM there are no large differences in conformation state of the frozen and with very high entropy segments or larger ensembles of them. With the load and temperature increasing there is a slight shift towards the higher temperatures and increased splitting of the melting peaks. With the load values increasing before everything is lost the low temperature component of the melting peak. It is difficult to define the contributions of the unidirectional influence of the heating and loading in the phases forming during the high temperature multiple melting proces. Sometimes these effects are slight and not quite as a regular visible trend but such is the nature of the processes of structural transformation, too. Much more impressive is the objects behavior in rubbery state in the whole temperature range of increasing segmental mobility and relaxation effects between the processes of glass transition and melting in comparison with the above discussed objects modified under linear heating.
Different types of defects and orientation inhomogeneity of PET fibers samples S1, S2 and S3 heat – mechanically treated at constant temperature of 95º º C and under constant gravimetric tensile loading of 120 MPa are present in Fig. 24.
In contrast to the rich on thermal effects relaxation area of the filaments rubbery state at the linear heated objects, here is observed fully stable samples behavior. The fibers modification under present conditions forms more homogeneous stable structure more compact (oriented) and poor of thermal effects during scanning proces.
3.4. Investigation of partially crystalline PET fibers simultaneous heat - mechanically modified at isothermal conditions and constant extension speed
After the examination of different variants of thermal treatment of PET fibers combined with different levels of constant gravimetric tensile loadings it was interesting to clarify the role of the mechanical treatment mode on the structure reorganization in the studied filaments. Therefore in the following variant of the thermal deformation experiment the studied partially crystalline PET yarns S1, S2 and S3 (Table 1) were subjected to extension at a constant speed under constant temperatures again in the same temperature range above the glass transition temperature from 80º º C to 95º ºC. In the present investigation PET bundles were annealed during ten minutes at temperatures 80º ºC, 85º ºC, 90º ºC and 95º ºC then were withdrawn with a constant rate of 0.1 m/min up to relatively elongation of 20 %, 40 % and 60 %. Immediately after the treatment the sample was removed from the furnace at room temperature. The structural developments of the so heat mechanically modified filaments were investigated using differential scanning calorimetry.
Received by DSC results for the samples S1, S2 and S3 show that at the gravitational load of the fibers the attenuation and the practical disappearance of the effects of softening and cold crystallization follow monotonic trend with the temperature increasing, while for the drawn samples the temperature dependence is weaker, seems secondary. For these samples has a stronger dependence of the observed effects from the mechanical influence, respectively from the withdrawal degree. In each panel of DSC curves is observed highly reduction of the intensity of the softening and cold crystallization running with increasing of the fibers elongation. At the highest temperature (95º ºC), these processes are less pronounced even at the small degrees of withdrawal but here also is observed damping of the effects, proportional to the extension degree increasing.
The appearance on effects of pre-melting in relaxation area around 190º ºC - 200º ºC (Fig. 25 and 26) in the fibers subjected to extension at a constant rate is very interesting. It is observed at all temperatures but is less pronounced in the intermediate temperatures of 85º ºC and 90º ºC. At the lowest temperature (80º ºC) the effect of pre-melting occurs at the largest deformation, respectively the largest structural reorganization. At the intermediate temperatures the effect is more slightly marked and is observed at the mean degree of the fibers elongation while at the highest temperature it occurs even at the smallest deformations. In addition all DSC curves with the observed pre-melting effects are steeper, with a larger gradient of the specific heat capacity increasing of the polymer structure with the temperature rise.
All this makes it possible to conclude that the forced modification of the studied PET fibers allows to form inhomogeneous and unstable stressed and overstressed structures which under certain conditions are thawed and melted during the DSC scanning. Similar entities could represent permanently stable in the time more stressed micro-localized areas, resulting in the observed during DSC measurement effects. For the gravitational loading similar effects are not monitored and this is reasonable. In the terms of this version of SHMM in the conditions of mutually coordinated influences and interactions of the mechanical affects the structural reorganization occurs more smoothly and slowly and turns off the formation and permanent freezing of overstressed regions. The DSC curves of both the filaments extended at a constant rate and filaments loaded at constant levels show beginnings of split of the melting peaks, which grow with the increase of the load and temperature.
It can be concluded that both the temperature and strain stress enable the structural reorganization and orientation of the polymer, but the forced withdrawal shows stronger orientation effects than the SHMM under constant loadings. Only concrete tests of the fibers strain-strength properties can arbitrate which samples are more optimally orientated and with less micro destructions.
Based on the analysis of the results obtained from the recent large-scale experiment it can draw the following conclusions:
There are studied PET fibers with different starting structure as amorphous as well partially crystalline samples with different pre-orientation;
Have been applied different regimes of thermal treatment as linear heating of the objects in a wide temperature range from 20º ºC to 200º ºC and annealing at constant temperatures in very important in terms of the structural changes temperature band from 80º ºC to 95º ºC, located close above the glass transition temperature;
The thermal treatments were accompanied from different levels of constant gravimetric tensile loadings and by withdrawal of the fibers to different levels at a constant extension rate;
The structural analyses of the investigated PET fibers subjected to SHMM show very important results concerning the role of the temperature and the applied strain stress and in particular:
the deformation behaviour of amorphous PET fibers subjected to linear heating from 20º ºC to 200º ºC show the role of the tensile loading in the management of the rival processes of fluid like deformation and strain induced crystallization;
significant increase of more than 30 % of the fibers degree of crystallinity in conditions of annealing in temperatures from 80º ºC to 95º ºC, with the strain stress increasing from 3 MPa to 6 MPa;
the received by DSC results for the partially crystalline samples S1, S2 and S3 show the role of the mechanical treatment mode on the running relaxation and phase transitions. At the gravitational fibers loading the attenuation and disappearance of the effects of the glass transition and cold crystallization follow monotonic trend with the temperature increasing, while for the drawn samples the temperature dependence is weaker, seems secondary. For these samples has a stronger dependence of the observed effects from the mechanical influence, respectively from the withdrawal degree.
The further steps are specific strain-strength tests that may arbitrate, which samples are optimally modified and such studies ahead.
The purpose of future research is by comparison between the gravitational load and different type of orientation drawing and thermal treatment to choose the optimal combination between them.
5. List of the used symbols
α- degree of the sample crystallinity
d- diameter of the single fiber
ε- value of the relatively bundle elongation
λ- wavelength of the radiation in the WAXS apparatus
L0- initial bundle length of the raw sample
L- final bundle length obtained after heat mechanically treatment
Δn- birefringence of single fiber
σ- applied to the objects tensile stress
VL- speed of the filaments formation