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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS
STOCKHOLM, SWEDEN 2021
Structural modifications of polyester fibres induced by thermal and chemical treatments to obtain high-performance fibres
Part A: Poly(ethylene terephthalate) fibres Part B: Poly(3-hydroxybutyrate) fibres KARTIKEYA SHARMA
KTH ROYAL INSTITUTE OF TECHNOLOGY
SCHOOL OF ENGINEERING SCIENCES IN CHEMISTRY, BIOTECHNOLOGY AND HEALTH
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SAMMANFATTNING
Del A: Polyetylentereftalat fibrer
I detta arbete presenteras olika metoder för att framställa monofilament av polyetylentereftalat (PET) (diameter: 30-50 µm) med en radiell gradient. Nyutvecklad Raman-spektroskopiteknik har använts för att kartlägga dessa inducerade radiella gradienter i t.ex. kristallinitet. På liknande sätt har FTIR-ATR teknik modifierats och anpassats för att studera ytegenskaperna hos dessa filament. Industriella filamentprover och egna smältspunna PET-filament har framgångsrikt modifierats med användning av olika termiska och kemiska behandlingar för att erhålla fibrer med förbättrade mekaniska egenskaper och minskad fibrillering. De strukturella förändringar som uppträdde i filamenten på mikroskopisk nivå karakteriserades med bl a infraröd analys, termisk analys, Raman-mikroskopi och röntgenteknik (SAXS och WAXD). Tester av fibrilleringsegenskaper utfördes av industriella partners med egenutvecklad teknik följt av testning av masterbatch-fibrer på en vävningssimulator. Resultaten i laboratorieskala avslöjade fibrernas strukturella anisotropi och radiella gradienter, vilka visade en minskad fibrillering med en viss inverkan på de mekaniska egenskaperna.
Del B: Poly(3-hydroxybutyrat) fibrer
Detta arbete presenterar studier av poly(3-hydroxybutyrat) (P3HB) fibrer med reversibla strukturförändringar. Tidigare studier har visat att kristallisationen hos P3HB fibrer i huvudsakligen sker i ortorombisk α-kristallform. Stress-anlöpning resulterar dock i en förändring i beteendet hos P3HB-materialet.
Strukturen hos P3HB fibrer består av amorfa och kristallina regioner samt en mesofas. Mesofasen antas vara belägen mellan α-kristallerna och uppträder som starkt orienterade bindningskedjor, s k “tie-chains”. Denna studie syftar till att observera effekten av stress-anlöpning på mesofasen och dess beroende av anlöpningsförhållandena. Förändringarna i mesofasen observeras med en anpassad och polariserad Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR) samt med Differential Scanning Calorimetry (DSC). Resultaten från ATR-FTIR visar att mesofasen är närvarande i spunna och högt stress-anlöpta fibrer, medan den är frånvarande i fibrer som är lågt stress-anlöpta. Mesofasen kan emellertid återupptas i lågt stress-anlöpta fibrer genom dragning. In situ ATR-FTIR användes för att studera förändringarna i materialbeteendet under en dragningsprocess för att observera periodiciteten i förekomsten av mesofasen. Det visade sig att förekomsten av mesofasen är en starkt reversibel process som observeras som en funktion av topparnas intensitet i ATR-FTIR.
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ABSTRACT
Part A: Poly(ethylene terephthalate) fibres
In this work, various methods to produce Poly(ethylene terephthalate) (PET) monofilaments (diameter: 30- 50µm) with a radial gradient are presented along with a newly developed Raman spectroscopy technique to map these induced radial gradients in e.g. crystallinity. On similar lines, FTIR-ATR technique has been modified and adapted to study the surface properties of these fine filaments. Industrial filament samples and in-house melt-spun PET filaments have been successfully modified using various thermal and chemical treatments to obtain fibres with improved mechanical properties and reduced fibrillation. The structural changes occurring in the filaments on the microscopic level were characterized using infrared analysis, thermal analysis, Raman microscopy and X-ray techniques (SAXS and WAXD) among others. The fibrillation properties were tested by the industrial partners using a technique developed in-house followed by testing of masterbatch fibres on a weaving simulator. Lab-scale results revealed the structural anisotropy and radial gradient maps of the fibres which also demonstrated reduced fibrillation with some impact on mechanical properties also being observed.
Part B: Poly(3-hydroxybutyrate) fibres
This work presents studies on poly(3-hydroxybutyrate) (P3HB) fibres with reversible structural changes.
Previously reported literature shows that crystallization of P3HB fibres takes place majorly in the orthorhombic α-crystal form. However, the stress-annealing results in a change of the material behaviour of P3HB. P3HB fibres compose of amorphous regions, crystalline regions and mesophase in their structure. The mesophase is supposed to be located in between the α-crystals of the material as highly oriented tie-chains.
This study targets to observe the effect of stress-annealing of the mesophase present in the P3HB fibres and its dependence on the annealing conditions. The changes in the mesophase content are observed with the help of a highly adapted polarized Attenuated Total Reflection Fourier Transform Infrared spectroscopy (ATR- FTIR) and Differential Scanning Calorimetry (DSC). The presented results from polarized ATR-FTIR show that the mesophase is present in as-spun and high stress annealed fibres while it is absent in fibres annealed with low stress. However, the mesophase can be re-obtained in low stress annealed fibres through tensile drawing. In-situ ATR-FTIR was utilized to study the changes in the material behaviour during a tensile drawing process to observe the cyclicity in the occurrence of the mesophase. It was found that the existence of mesophase is a highly reversible process observed as a function of the peak intensities of the polarized ATR- FTIR spectroscopy.
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FOREWORD
I would like to express my deepest gratitude to my master’s thesis supervisor at EMPA, Dr Edith Perret, for giving me this opportunity to work on these exciting projects with industrial collaborations. I would like to thank her for constantly being available for discussions and for guiding me towards better interpretation of results. Despite the pandemic, she took extra efforts to ensure my relocation to Switzerland and also made sure that I am able to perform my thesis comfortably in the group and the department. At the same time, I would also like to thank Dr Rudolf Hufenus, without whom I wouldn’t have had this opportunity of working at EMPA. I would like to acknowledge my colleagues from my time at EMPA for their support during my experiments. I would particularly like to thank Benno Wust and Markus Hilber for training me and providing me unlimited access to various instruments, including the melt spinning plant and the 3D printer.
I would like to thank my examiner at KTH, Professor Lars Berglund, for his constant support while carrying out my thesis. I very much appreciate his patience and promptness during our electronic conversations and for providing all the necessary documents which were required for the various permits and applications. His research acumen, scientific inclination and exceptional outlook on the topic helped me to go beyond the conventional approaches and obtain interesting results. I would also like to acknowledge the responsible team at the academic office of the Fibre and Polymer Technology division at KTH for their support and encouragement to pursue research during an unconventional timeline and in another country.
Finally, I would like to thank my family, both, in India and Switzerland, for their never-ending support and continuous encouragement towards my academic activities. This work and outcome would not have possible without their engagement during the highs and lows during the period of my stay.
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Part A: Poly(ethylene terephthalate) fibres
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Structural modifications of polyester fibres induced by thermal and chemical treatments to obtain high-performance fibres- Part A: Poly(ethylene terephthalate) fibres.
Kartikeya Sharma1,2
1 KTH Royal Institute of Technology, Stockholm 11416, Sweden.
2 Laboratory for Advanced Fibers, Empa, Swiss Federal Laboratories for Materials Science and Technology, Lerchenfeldstrasse 5, 9014 St. Gallen, Switzerland.
Abstract:
In this work, various methods to produce Poly(ethylene terephthalate) (PET) monofilaments (diameter: 30- 50µm) with a radial gradient are presented along with a newly developed Raman spectroscopy technique to map these induced radial gradients in e.g. crystallinity. On similar lines, FTIR-ATR technique has been modified and adapted to study the surface properties of these fine filaments. Industrial filament samples and in-house melt-spun PET filaments have been successfully modified using various thermal and chemical treatments to obtain fibres with improved mechanical properties and reduced fibrillation. The structural changes occurring in the filaments on the microscopic level were characterized using infrared analysis, thermal analysis, Raman microscopy and X-ray techniques (SAXS and WAXD) among others. The fibrillation properties were tested by the industrial partners using a technique developed in-house followed by testing of masterbatch fibres on a weaving simulator. Lab-scale results revealed the structural anisotropy and radial gradient maps of the fibres which also demonstrated reduced fibrillation with some impact on mechanical properties also being observed.
Keywords:
PET fibres; Radial gradient; Chemical modification, Thermal annealing, High-performance fibres.
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List of Figures
Figure 1. Image showing the fibrillated fibre ends from the fibrillation test.
Figure 2. Schematic showing the process of simultaneous drawing and annealing of the fibres.
Figure 3. Image showing the action of plasticizer onto the fibre morphology.
Figure 4. Illustration of the drawing setup for drawing over the heated plate at a distance of 2 mm.
Figure 5. Illustration of the drawing process employed using a heated furnace.
Figure 6. Illustration of the drawing process involving the use of a heated godet.
Figure 7. Illustration of the drawing process involving the use of plasticizers and heating plate.
Figure 8. Schematic of the melt spinning plant used for manufacturing the fibres used in the study.
Figure 9. Thermograms of DR1, DR2 and DR4 obtained from DSC analysis.
Figure 10. FTIR- ATR spectra overlap of DR1, DR2 and DR4 samples.
Figure 11. Different peaks from the FTIR-ATR spectra of DR1, DR2 and DR4 samples.
Figure 12. Raman spectra of DR1 and DR4 samples focused between 800cm-1 and 1800cm-1. Figure 13. Raman peak fits in the different regions for DR1, DR2 and DR4 samples.
Figure 14. Processed Raman data showing the changes in intensity ratios upon been drawn, a representative of changes in degree of crystallinity.
Figure 15. Representation of the structural gradient obtained from the Raman analysis of the different PET samples.
Figure 16. DSC thermograms for different samples undergoing thermal annealing in furnace.
Figure 17. FTIR-ATR spectra of the different samples annealed thermally through the furnace.
Figure 18. Results from the fibrillation test for the furnace treated samples.
Figure 19. Tensile measurements of the samples treated in the furnace along with the standard samples.
Figure 20. DSC thermograms obtained for godet treated samples.
Figure 21. FTIR-ATR spectra of samples treated using a heated godet.
Figure 22. Peak intensity ratio plots for different samples.
Figure 23. Results from the fibrillation test of the godet treated samples.
Figure 24. The tensile plot of samples treated with godet at speed of 760 rpm.
Figure 25. Stress-strain plots of differently treated samples on the godet and the heating plate.
Figure 26. FTIR-ATR spectra of samples treated with different plasticizers for 10 minutes.
Figure 27. FTIR spectra for fibres treated with different concentrations of a) DGDB and b) BSA plasticizers.
Figure 28. DSC thermograms of samples treated for 10 minutes.
Figure 29. DSC thermograms of samples treated with different concentrations of BSA.
Figure 30. Tensile test plots for the a) melt spun fibres with plasticizers and b) hot drawn fibres treated with plasticizers.
Figure 31. Results from the fibrillation tests of the samples melt spun using plasticizers.
Figure 32. Microscopic images from the fibrillation test of sample a)193 and b) 10%BSA.
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List of Tables
Table 1. Properties of the polymers used for the synthesis of the fibres.
Table 2. Properties of the fibres used in the modification process.
Table 3. Details of the plasticizers and crosslinkers used for fibre modifications.
Table 4. The parameters of the offline drawing over a heating plate. The number of windings is given in brackets.
Length of heating plate= 40 cm.
Table 5. The parameters of the drawing process employed using a heated furnace. Length of the furnace = 100 cm.
Table 6. Parameters of the heated godet assisted drawing and thermal annealing process. Circumference of godet = 40 cm.
Table 7. Standard values of operation for the fibrillation test.
Table 8. Thermal properties and crystallinity values for the different samples.
Table 9. Relevant PET FTIR wavenumber assignments.
Table 10. Peak assignment for the Raman spectra of DR1, DR2, and DR4.
Table 11. Summary of data obtained from DSC analysis of samples.
Table 12. Treatment details and contact time values for heating of the fibres.
Table 13. Thermal properties and crystallinity values for godet treated samples.
Table 14. Thermal properties and crystallinity values for samples treated with plasticizers for 10 minutes.
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1. Introduction:
In recent years, the global annual textile fiber production exceeded 82 million tons, of which around 60%
consisted of synthetic materials. Among synthetic fibres, PET fibres lead the worldwide production. The Global Polyester Market Volume is Expected to Reach 107.84 Million Tonnes in 2023, Growing at a CAGR of 5.60% [1]. In 2019, the U.S. production volume of polyester fiber amounted to a total of some 1.28 million metric tons. The unique properties of this fibre arising due to the presence of aliphatic and aromatic parts in the macromolecular chains allow this fibre to be utilized in a wide range of applications [1]. The predominance of PET fibres in the synthetic fibre market is particularly due to its good end use properties, economy of production and the ease of physical and chemical modification [2].
PET is produced commercially by the dimethyl terephthalate (DMT) and terephthalic acid (TPA) processes, proceeding via condensation polymerization reaction [3]. Using DMT and TPA yield two different kinds of PET materials, having different properties. The most common method of manufacturing PET fibres is by melt spinning taking place at temperatures close to 285°C [1]. At this temperature, the PET viscosity is typically quite high, around 3000 Poise for high molecular weight PET. The production speed for melt spinning can be very high, e.g. 4000 m/min. The emerging thin filaments from the spinneret are solidified via quenching and then drawn at a temperature above Tg to improve the orientation of polymer chains and increase strength.
PET fibres are typically semi crystalline and thus have a combination of crystalline and amorphous regions [4]. The presence of crystalline regions in the polymer structure allows the fibre to attain high mechanical properties and to be used as a high-performance material while the amorphous regions allow for the colouration and accessibility properties of the material [5]. For its application as a textile fibre, PET fibres with a maximum crystallinity of 35-40% are used to obtain an optimum ratio of strength to aesthetic functionality.
Due to this high degree of crystalline regions present in the structure, certain problems, like fibrillation, arise during the application of these fibres [6].
Fibrillation is a phenomenon which is observed very commonly for synthetic fibres when they are exposed to bending stresses or abrasion. Fibrillation can be defined as the splitting up of microfibrils or smaller fibres from the surface of the fibre bundle or a filament [7]. The diameter of these separating fibrils can range from a few nanometers to a few micrometers, depending on the fibre structure. This splitting phenomenon arises due to the weak cohesive forces between the fibrils [8]. When the fibres are processed to have a high crystallinity and a highly oriented structure, the inter-fibril linkages are weakened due to the presence of some chains that are under high stresses, leading to low inter fibril shear strength [9]. Hence, when these fibres are subjected to abrasion or bending around edges, some weakly linked fibrils tear off and fibrillate.
Nevertheless, polymeric fibres with high orientation and crystallinity are very sought after as these properties are the prerequisite for having high tensile strength [10, 11].
In order to overcome this problem of fibrillation, various techniques have been explored by researchers. The simplest way is to attain a compromise between the tensile strength and the degree of orientation of the fibres. One method that has been explored makes use of crosslinkers, which penetrate into the amorphous regions between the chains and bind them together, preventing the possibility of splitting [8,12]. Other methods include the application of plasticizers to improve the mobility and re-organisation of chains to
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release the stresses that are built up in the fibers. Also, thermal annealing of the fibres can alter the orientation of the chains and the crystallinity of the fibres. Another method, which has been exploited, is the production of bicomponent fibres, where the sheath material has a high fibrillation and abrasion resistance while the core material provides the high tensile strengths.
Radial gradient of crystallinity in fibres has been a topic of extensive research over the period of time.
Gradient polymer blends are truly attractive materials since their particular morphology allows them to develop desired properties in certain regions of the material [13-15]. The impact of a gradient morphology is not necessarily limited to the bulk, but can also influence the surface properties. Based on this aspect of gradient structures, introduction of radial gradients has been studied to improve the fibrillation properties of synthetic fibres. Introduction of structural gradients can be implemented most easily by melt spinning of a bicomponent fibre having a highly crystalline material in the core, providing the mechanical properties and strength while the sheath material is a less crystalline material in order to prevent fibrillation [16, 17]. This method involves very careful selection of materials in order to form a highly stable and adhesive interface between the core and the sheath material. Although the melt spinning of bicomponent fibers is innovative and highly optimized, it does not come without risk. The general concern with bicomponent fibres has been the possibility of interfacial debonding between the core and sheath materials, causing internal slippage and failure of the material [18]. Furthermore, bicomponent melt spinning is technically much more challenging and is also quite expensive.
In order to overcome the general risk of multi-polymer material, bonding failure and to reduce costs, different approaches were examined for the modification of fibres to reduce fibrillation tendency. These approaches involved the application of heat and plasticizing chemicals to achieve gradients in the structure of the monofilaments.
1. 1. Problem description
For a confidential application, it was required to develop high-performance monocomponent PET micro- filaments with high tensile strength and high fibrillation resistance. The industrial competitors for the intended application currently make use of bicomponent fibres to achieve the desired properties. In order to achieve a cost-effective production method of high-performance fibers, similar properties were targeted to be achieved with the modification of monocomponent fibres. This was done by inducing crystallinity gradients in their structure such that the core of the fibres is highly oriented and crystalline while the surface of the fibres attains a lower crystallinity and less-oriented chains to prevent fibrillation, which occurs during the application of the fibers.
Figure 1. Image showing the fibrillated fibre ends from the fibrillation test.
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1. 2. Objectives
In order to modify the monocomponent PET fibres, multiple techniques were investigated. The basic concept behind each technique was to obtain differential structural properties in the core and the surface of the fibres. The idea behind this approach was that having a less crystalline and oriented structure on the surface would lead to reduced fibrillation while having a highly crystalline and oriented core would provide the fibres with the high-performance properties. Since the fibres were monocomponent, problems arising due to the presence of an interface between two polymeric materials, as in the case of bicomponent fibres, would not arise. The modification approaches are divided into two sections: Thermally induced modifications and chemically induced modifications.
1. 2. 1. Annealing
Thermally induced modifications involved annealing the fibres thermally through different setups to induce a gradient in the structure of the fibres.
The simultaneous stretching and annealing of fibres (Figure 2) is expected to induce a gradient temperature profile in the fibre and thus lead to the development of a gradient structure [19, 20]. Different thermal gradient profiles are achieved, depending on whether the melt-spun 'hot' fibre is drawn directly after spinning (online drawing) or whether the 'cold' as-spun fibre is reheated and drawn in a second step (offline drawing). In the latter case, the thermal gradient is intended to run from the outside to the inside of the fibre with a decreasing temperature effect, causing molecular re-orientations in the core and sheath regions. The melt-spun fibres would experience a reduction of draw-induced molecular orientation and crystallization [21]
in its outmost region during the modification. The strain-induced crystallization would mainly occur in the core, since the core would be colder for a longer time. As a result, the fibre’s sheath is becoming soft, ductile and flexible, lowering the tendency of fibrillation [22]. In this work, the application of different heat sources during offline drawing have been examined: heated plates, heated godets and a heated chamber. The exposure time to the heat were varied and temperatures ranged between 90oC and 170oC in order to achieve different thermal gradients.
Figure 2. Schematic showing the process of simultaneous drawing and annealing of the fibres.
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1. 2. 2. Chemical modifications
Chemical modifications were implemented with the help of plasticizers (Figure 3). When fibres consisting of highly oriented polymers chains are exposed to strain, the internal reorganization of their macromolecular chains does not take place in order to be able to relieve the stresses, resulting in fibrillation [23]. Plasticizer molecules are expected to lubricate and occupy the intermolecular spaces between these polymer chains in order to allow such a reorganization in the structure. This is achieved by reducing secondary forces between the polymer chains and thus decreasing the energy required for molecular motion [24, 25]. Plasticizers having different chemistries and functional groups were identified to be used for PET and were used in varying concentrations and varying contact time to trace the penetration of the plasticizer into the fibre while also analyzing the changes brought about in the structure. Theoretically, the plasticizers either penetrate into the fibre structure causing the rearrangements in the chains or remain on the surface up to a certain depth causing the same effect on the surface.
Figure 3. Image showing the action of plasticizer onto the fibre morphology.
2. Materials
The fibres used throughout the experiments were either melt-spun through an in-house assembly at Empa St Gallen or were provided by the industrial partners, Monosuisse AG and Sefar AG. The fibres were melt-spun using two different polymers depending on whether the fibres had to be treated in a one-step method (online drawing) or a two-step method (offline drawing). Polymer P1 (Invista RT20 PC110) was a polyester that was prepared from DMT as the starting material and the pellets for this were provided by Monosuisse AG. Polymer P2 (Grisuten 115), also supplied by Monosuisse AG, was synthesized with PTA as the raw material. The specifications for these materials are presented in Table 1. Filaments were melt-spun from these polymers with different diameters and fineness with two different drawing processes. The fibre specifications are tabulated in
Table 2.
Table 1. Properties of the polymers used for the synthesis of the fibres.
Label Material Monomer Instrinsic visocisty (dL/g)
P2 Grisuten 115 PTA 0.99
P1 Invista RT20 PC110 DMT 0.94
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Table 2. Properties of the fibres used in the modification process.
Label Material Drawing Fineness (dtex) Diameter (µm) Colour
229 Grisuten 115 As-spun 61.0 73.2 Yellow
211 Grisuten 115 2-step 14.0 35.0 Yellow
193 RT20 PC110 1-step 14.0 35.0 White
DR1 RT20 PC74 As-spun 82.3 84.2 White
The melt-spun fibres were supplied with a spin finish, E904, that was applied onto the fibres as a protective lubricant.
The fibres provided by the industry were already prepared with E904 and it was also used during the in-house melt- spinning process. Five plasticizers were utilized to incorporate the structural gradients in the fibres: Di(propylene glycol) dibenzoate (DGDB), Biphenyl (BP), N-Butyl benzenesulfonamide (BSA), N-Methyl-p-toluene sulfonamide (MTSA) and Diphenyl sulphone (DPS) were all procured from Sigma-Aldrich Chemicals Pvt Ltd. Two crosslinkers, trivinylphosphine oxide (TVPO) and pentaerythritol triacrylate (PETA) were also used in the process. TVPO is a chemical developed in- house while PETA was procured from Sigma-Aldrich Chemical Pvt Ltd as well. Azobis-isobutyronitrile (AIBN), to be used as initiator, along with technical grade Toluene and Acetone was also sourced from the same company. The specifications for plasticizers and crosslinkers are summarized in Table 3.
Table 3. Details of the plasticizers and crosslinkers used for fibre modifications.
Chemical name Label % Purity State of chemical
Di(propylene glycol) dibenzoate DGDB 75 Liquid
N-Butyl benzenesulfonamide BSA 99 Liquid
N-Methyl-p-toluene sulfonamide MTSA 98 Solid
Diphenyl sulphone DPS 97 Solid
Trivinylphosphine oxide TVPO 100 Liquid
Pentaerythritol triacrylate PETA 100 Liquid
2. 1. Experimental setups:
2. 1. 1. Offline drawing and annealing:
Fibers for fundamental study:
In order to understand the rearrangement processes occurring during the simultaneous heating and drawing of fibres and to establish a method to analyze the resulting structural radial gradients, the experimentation was started with a fundamental study. In this study, differently drawn white fibers have been produced and subsequently analyzed with DSC, FTIR-ATR and Raman mapping. For this purpose, the white low-viscosity fibre, DR1, was drawn with a modifiable drawing setup constructed at Empa (Figure 4).
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Figure 4. Illustration of the drawing setup for drawing over the heated plate at a distance of 2 mm.
The setup consisted of three godets attached in line with a heating plate (170 °C) situated between the last two godets.
Drawing was carried out in three different steps. The first minor cold drawing occurred between godet 1 and godet 2.
Following this, was the major drawing over the heated plate (2mm above the plate, not in direct contact) and the final minor drawing occurred between godet 3 and the winder. Fibers with two different draw ratios (DR2, DR4) were produced, the godet speeds and temperatures are given in Table 4.
Fibers for annealing study:
In order to understand the impact of different heating sources on the mechanical properties and structure of the fibers, an annealing study has been performed. The same offline drawing setup for as-spun fibres has been used as for the fundamental study, but using different heating sources. The general setup also consisted of three godets attached in line or at an angle but with an exchangeable main heating source, which was either a heating plate, a heated chamber or a hot godet depending on the used annealing method. As in the fundamental study, the drawing was also carried out in two steps, a first minor cold drawing and a major hot drawing. Godet 1 was always maintained at room temperature while Godet 2 was heated to 95oC. The main heating source (plate, furnace or godet) was heated to different temperatures in order to vary the thermal gradients in the fibers and the number of windings on the godets were also varied.
The first goal was to reproduce the offline drawing method, which is applied by the industry and to achieve similar mechanical properties of offline drawn fibers as those of industrial fibers. Thus, the first offline drawing approach involved the use of a heating plate (170°C), which was directly in contact with the fibre (Figure 4). This direct contact with the high temperature was expected to cause higher rearrangements on the surface of the fibre, creating a gradient in the structure. The drawing parameters and settings were chosen to be very similar to the ones used by the industry.
First, the as-spun yellow fiber, 229, with high-viscosity PET was drawn with different draw ratios in order reproduce the mechanical properties of the offline drawn fiber 211. The final settings that led to almost identical properties as fiber 211 are summarized in Table 4 for the fiber P170-762.
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Table 4. The parameters of the offline drawing over a heating plate. The number of windings is given in brackets. Length of heating plate= 40 cm.
Fiber label Godet 1 (at 20 °C) speed (m/min)
Godet 2 (at 95 °C) speed (m/min)
Heating plate temperature (°C)
Godet 3 (at 20 °C) speed (m/min)
Winder speed (m/min)
Draw ratio
Original as-spun fibre for drawing
DR2 70 (6w) 71 (2w) 170 140 142 2.03 DR1
DR4 70 (6w) 71 (2w) 170 280 285 4.07 DR1
P170-760 163 (1w) 164 (8w) 170 761 (8w) 762 4.67 229
The next annealing method involved exposing the fibres to hot air (100-130oC) during the hot drawing process, as shown in Figure 5. The hot air was utilized to increase the heat transferred to the fibre at lower temperature to cause the structural changes. In this method, the furnace was placed between godet 2 and godet 4 where the major drawing took place. Minor drawings also took place between godet 1 and godet 2 and between godet 4 and godet 5. Three different draw ratios were employed in this method while the temperature of the hot air in the furnace was also varied.
The parameters of this drawing process are summarized in
Table 5.
Figure 5. Illustration of the drawing process employed using a heated furnace.
Table 5. The parameters of the drawing process employed using a heated furnace. Length of the furnace = 100 cm.
Label Godet 1 (20oC) 1 winding
Godet 2 (95oC) 8 windings
Furnace temperature
Godet 4 (20oC) 8 windings
Godet 5 (20oC) 1 winding
Winder speed (m/min)
DR Original as- spun fibre for drawing Speed
(m/min)
Speed (m/min)
Speed (m/min)
Speed (m/min)
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F130-760 163 164 130 760 761 762 4.67 229
F112-782 163 164 112 780 781 782 4.80 229
F130-815 163 164 130 813 814 815 5.00 229
F135-322 70 71 135 320 321 322 4.60 229
The final approach employed the use of another heated godet for the drawing and thermal annealing. The setup employed has been shown in Figure 6. Instead of a furnace or a heating plate, a godet was used, which was maintained at a temperature of 170oC. This direct heating in combination with higher cooling times was suspected to produce higher gradients in the structure due to the increased contact time with the heated godet and increased cooling time before being wound in the winder. In this method, the number of windings around the heated godet and the following godet were varied along with the value of the draw ratio in between the godets with the overall draw ratio being maintained at 4.67. Table 6 provides the parameters for this method if drawing and thermal annealing.
Figure 6. Illustration of the drawing process involving the use of a heated godet.
Table 6. Parameters of the heated godet assisted drawing and thermal annealing process. Circumference of godet = 40 cm.
Label Godet 1 (20oC) 1 winding
Godet 2 (95oC) 8 windings
Godet 3 (170oC)
Godet 3 Godet 4 (20oC) 8 windings
Winder speed (m/min)
DR Original as-spun fibre for drawing Speed
(m/min)
Speed (m/min)
Speed (m/min)
windings Speed (m/min)
G170-760(1w) 163 164 760 1 761 762 4.67 229
G170-760(3w) 163 164 760 3 761 762 4.67 229
G170-760(5w) 163 164 760 5 761 762 4.67 229
G170-760(7w) 163 164 760 7 761 762 4.67 229
G170-760(10w) 163 164 760 10 761 762 4.67 229
G170-700(1w) 163 164 700 1 761 762 4.67 229
G170-700(3w) 163 164 700 3 761 762 4.67 229
G170-700(5w) 163 164 700 5 761 762 4.67 229
G170-700(7w) 163 164 700 7 761 762 4.67 229
G170-600(1w) 163 164 600 1 761 763 4.67 229
G170-600(3w) 163 164 600 3 761 763 4.67 229
G170-600(5w) 163 164 600 5 761 763 4.67 229
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G170-600(7w) 163 164 600 7 761 763 4.67 229
2. 1. 2. Offline drawing and chemical modification:
In order to modify the fibres through chemical means, solutions of varying concentrations of plasticizers were prepared in Acetone and E904. Acetone was used to prepare the solutions because it was expected that the fibre would swell when exposed to acetone and this would make it easier for the plasticizer to penetrate the fibre [26]. Solutions of 1 wt.% and 10 wt.% prepared in E904 were applied to the fiber before the first godet in the drawing setup with the heating plate in order to observe the combined effect of thermal annealing and chemical restructuring. The illustration for the drawing process involved in shown in Figure 7. Additionally, solutions with higher concentrations of 10 wt.%, 25 wt.% and 40 wt.% of plasticizers in acetone were used to perform a fundamental study on the actual structural changes occurring on the microscopic level in the polymer fibre.
Figure 7. Illustration of the drawing process involving the use of plasticizers and heating plate.
2. 1. 3. Online drawing and chemical modification setup:
Solutions with 1 wt.% and 10 wt.% of plasticizers were applied during the melt spinning of fibres. These solutions were prepared in E904 since the fibres to be used in the intended application will be prepared with the spin finish E904. The melt spinning of the fibres was carried out at the pilot plant setup at Empa, St. Gallen. The schematic of the setup is shown in Figure 8. The melt spinning was carried out in N2 environment to prevent any moisture absorption. The setup comprised of a single screw extrudes with L/D ratio of 25. The diameter of the spinneret used was 0.7 mm. The setup also comprised of multiple drawing godets and solution applicators supplied with different chemicals involved in the modification process. The solutions of plasticizers were applied on the fibres via the first solution applicator to combine the effect of plasticization and thermal treatment.
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Figure 8. Schematic of the melt spinning plant used for manufacturing the fibres used in the study.
3. Methods:
3. 1. Differential scanning calorimetry (DSC)
Average crystallinity values and thermal properties of all fibers were determined using differential scanning calorimetry (DSC). Measurements were performed on the instrument (DSC 214 Polyma, Netzsch, Selb, Germany) in a nitrogen atmosphere (40 mL/min). The fibers were cut into small pieces (~ 2 mm long) and in each case about 5-10 mg of cut fibers were first heated from 25 °C to 300°C followed by a cooling step from 300 °C to 25 °C using a ramping rate of 10
°C/min. The data was analyzed using a DSC software (NETZSCH Proteus- thermal Analysis, Version 7.1.0, Selb, Germany) in order to extract the crystallinity of individual samples.
3. 2. Attenuated Total Reflection Fourier Transform Infrared spectroscopy (ATR-FTIR)
The PET monofilaments were analyzed with ATR-FTIR. ATR-FTIR spectra were recorded with a Bruker Tensor 27 FTIR spectrometer (Bruker Optics, Ettlingen, Germany), using a single reflection attenuated total reflectance (GladiATR™) accessory from Pike Technologies (Fitchburg, Wisconsin, United States). The FTIR spectrometer uses a mid-infrared (MIR) Globar source and a narrow-band mercury cadmium telluride (MCT) detector. The ATR accessory is equipped with a monolithic diamond ATR crystal. The infrared light is guided with several optical parts to the sampling surface with an angle of incidence of 45°. The depth of penetration of the infrared light into the PET sample can be calculated using the following equation [27]:
where λ is the wavelength of interest, and n1, n2 are the refractive indices of the ATR crystal and the PET polymer (dispersion is neglected), respectively. For wavenumbers ranging from 1470 to 840 cm-1, (λ=0.00068 cm to 0.00119 cm), the calculated penetration depth, Δz, varies from 1.5 to 2.7 µm.
A special holder was built to reproducibly position fibers (always oriented along the path of the infrared light) on the sampling stage. A reproducible contact pressure between the ATR crystal and the sample was ensured by handling the pressure clamp of the ATR system in the exact same way for each measurement. Absorbance spectra spanning wavenumbers between 4000 and 600 cm-1 were collected with a spectral resolution of 2 cm-1.For each spectrum a total
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of 32 scans has been taken and averaged. To minimize differences between spectra due to baseline shifts, the spectra were cut in the range of 4000 to 700 cm-1, due to high noise below 700 cm-1, and the baselines were subsequently subtracted by using a concave Rubber band algorithm with 10 iterations using OPUSTM software (Version 8.5, Bruker AXS, Karlsruhe, Germany). Additional data analysis such as normalization and peak fitting have been performed with specifically developed python codes.
3. 3. High-resolution Raman mapping
Raman spectra were acquired using a WITec Alpha 300 R confocal Raman microscope (WITec GmbH, Ulm, Germany) in backscattering geometry at Empa, Dübendorf, Switzerland. As an excitation source, a blue laser with 488 nm wavelength was used. The light was focused onto the embedded sample using a 100 × objective with a numerical aperture of 0.9, resulting in a diffraction limited in-plane laser spot size of < 1 µm. The confocality of the Raman microscope limits the focal depth to approximately < 1 µm. The Rayleigh scattered light was blocked by a notch filter.
The backscattered light was coupled to a 300 mm lens-based spectrometer with a grating of 1800 g/mm for all filaments. The spectrometer is equipped with a thermoelectrically cooled CCD. Raman spectra were acquired with a set laser power of 5 mW with an integration time of 0.4 s for DR1, DR2 and DR4.
PET fibers were embedded in a resin hardener (Epoxi cure 2, Buehler, USA) in order to measure Raman maps of polished fiber cross-sections. Fibers were wound on a sample holder and subsequently embedded under vacuum in order to avoid air inclusions. The samples were cured overnight and further grinded and polished several times in order to obtain a smooth and transparent surface for subsequent Raman spectroscopy measurements. Pictures of the embedding process are shown in the supplemental information.
The embedded samples were mounted on a piezo stage and maps of fiber cross-sections were acquired by scanning the sample through the laser. The raw spectra have been treated with a cosmic ray removal procedure and a background was subtracted using a moving shape with radius 100 cm-1 in order to remove signatures of photoluminescence. All corrected Raman spectra were subsequently binned by averaging over 4 spectra (2x2), spanning an area of 1 µm2. Before fitting, the binned spectra were all normalized by the intensities of the 1291 cm-1 Raman band. This normalization was necessary in order to account for changes in the absolute intensity due to a slightly bent fiber surface, which we have observed with atomic force microscopy (supplemental information). Spectral analysis was done with specially developed python codes, where Raman peaks of interest were fit with one or two Pearson VII functions and a linear background.
3. 5. Tensile strength measurement
The stress-strain behavior of the fibers was evaluated using the Textechno STATIMAT ME+ (Herbert Stein GmbH, Germany) tensile tester with a 10 N load cell, following ASTM D2256. Twenty samples of each filament with a clamping length of 100 mm were measured with a pre-tensioning force of 0.2 cN/tex at a constant deformation rate of 100 mm/min. The data obtained was further treated using specifically designed Python codes.
3. 6. Fibrillation properties
The fibrillation tendency of fibres was tested using a standard setup devised by Monosuisse AG. The setup is based on a simple principle in which fibre bundles are hit against a cylinder. Hereby, the fibre bundle is attached to a fixture, which is connected to a motor, and then repeatedly hit against the cylinder in a circular motion. The length of the fibre, the speed and the distance d between the device and the impact cylinder are fixed to certain values which are tabulated in Table 7. The interpretation of the data from the fibrillation test was done by visual inspection of the bundle ends
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with microscope images giving marks from 0 to 8, with 0 showing almost no fibrillated fibers (0-10%) and 8 showing a lot of fibrillated fibres (76%-100%) and many fibrils towards the bundle ends.
Table 7. Standard values of operation for the fibrillation test.
Time (min) Speed (rpm) Distance (mm) Excess length (mm)
2 × 3 2348 62 12
4. Results and Discussions
4.1 Fundamental study 4.1.1. DSC analysis
The average crystallinity of the differently drawn fibres was determined using DSC. Figure 9 shows the thermograms obtained from the first cycle of the analysis, representing the structural changes occurring in the fibres.
Figure 9. Thermograms of DR1, DR2 and DR4 obtained from DSC analysis.
From Figure 9, it can be observed that as the draw ratio is increased, the melting peak of the fibre becomes sharper while the peak of cold crystallization starts to reduce. For the DR4 sample, the cold crystallization peak is absent, representing the transition from an excess of amorphous regions to a crystalline phase. A cold crystallization peak in the first heating curve is typically observed for fibers that have a high amount of
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amorphous phase, which can easily crystallize upon heating. Such a cold crystallization peak is however absent in highly drawn filaments of semi-crystalline materials, since they already have a high crystallinity to start with. The amorphous material which is located between the crystals in these fibers has most-likely not enough time to further crystallize during the first heating cycle.
The calculated percent crystallinity from the DSC curve of the as-spun fiber (DR1) shows that this fiber is amorphous (𝜒 ~8%). As expected, the degree of crystallinity increases with increasing draw ratio in samples DR2 and DR4 due to strain-induced crystallization [28]. Once the polymer reaches its glass transition temperature, the chains become more flexible and hence are able to unfold under stress. The randomly coiled and entangled chains begin to disentangle and straighten, leading to a more crystalline material [29].
The increase in crystallinity from DR1 to DR4 can be seen by the decrease in the enthalpy of cold crystallization, and by the increasing sharpness of the melting peak. The thermal properties are presented in Table 8.
Table 8. Thermal properties and crystallinity values for the different samples.
Fiber label Tcc
(°C)
ΔHcc (J/g)
Tm
(°C)
∆𝐻𝑚𝑃𝐸𝑇 (J/g)
𝜒 (%)
DR1 143.2 29.3 ± 2.1 257.9 40.4 ± 3.6 7.9 ±1.2
DR2 121.0 25.3 ± 2.0 257.6 41.2 ± 3.5 11.4 ± 1.6
DR4 - - 254.7 51.5 ± 0.8 36.8 ± 0.6
4.1.2. FTIR- ATR analysis
FTIR was utilized to understand the structural properties of the fibres at the surface. Hence, samples DR1, DR2 and DR4 were studied using this technique and their FTIR-ATR spectra are shown in Figure 10. These spectra were normalized by dividing the measured absorbance by the maximum absorbance in the range from 1225 to 1275 cm-1. This peak arises from ring and C-O stretching. The corresponding peak in Raman analysis is observed at 1291 cm-1, which has been used to normalize the Raman spectra.
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Figure 10. FTIR- ATR spectra overlap of DR1, DR2 and DR4 samples.
FTIR studies on PET have reported that IR bands near 1471cm-1 and 1340 cm-1 increase in intensity during crystallization due to trans conformations of the ethylene glycol segment in crystalline PET [30-32]. Bands having wavenumbers close to 1453cm-1 and 1370 cm-1 have been associated with the gauche conformation of PET, and are typically reducing in intensity during crystallization [32]. These infrared bands representing the gauche and trans conformers of different functional groups have been shown in Figure 10 as the highlighted grey bands. These gauche conformations are only present in the amorphous regions of the polymer, while trans conformations are present in both, crystalline and amorphous regions of the polymer [33]. The important FTIR bands are listed in Table 9. These characteristic infrared bands have been used to evaluate differences in surface crystallinity between individual samples, by analyzing the individual peak height via curve fitting. As expected, upon drawing the peak heights of gauche ethylene glycol conformations are reducing, while those of trans are increasing [34]. From Figure 10, various differences can be observed for the DR1, DR2 and DR4 samples. The intensity of the peak representing the carbonyl functional group at 1714 cm-1 increases with the increase in the draw ratio of the fibres [35]. Similarly, the increase in the intensities of the peaks corresponding to the trans conformers can be seen to increase at 1471 cm-1 and 1340 cm-1 upon increasing the draw ratio of the fibres. A similar but reverse effect is observed for the peaks representing the gauche conformers of the PET structure at 1453 cm-1 and 1370 cm-1. Further, changes corresponding to the presence of other groups in the structure of PET can also be observed at other wavenumbers. Peak at wavenumber 1578 cm-1 which corresponds to the C-C stretching shows can increase in intensity upon decreasing the draw ratio [36]. The same effect is also observed for peaks at 1505 cm-1 (C- H in-plane stretching) and 1043 cm-1 (C-O gauche stretching) [35]. However, increase in peak heights upon increasing the draw ratio has been observed at 1118 cm-1 and 972 cm-1, apart from the mains peaks under consideration. Both these peaks correspond to the trans stretching of the C-O functional groups [37].
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Table 9. Relevant PET FTIR wavenumber assignments.
1471 CH2 bending, trans conformers
1453 CH2 bending, gauche conformers
1370 CH2 wagging gauche conformers of the ethylene glycol unit 1340 CH2 wagging of trans conformers of the ethylene glycol unit
1043 C-O stretching, gauche conformers
972(979) C-O stretching, trans conformers
Based on the differences arising in the peak heights and intensities, attempts were made to study the surface crystallinity of DR1, DR2 and DR4 samples using these peak intensities. In Figure 11a, the changes in peak values with regard to draw ratio can be observed. The intensity increases significantly for the peak corresponding to the trans conformer from DR1 to DR4 samples. Similar effects have also been observed in the region of 1400 cm-1 – 1300cm-1 as shown in Figure 11b. These peak heights were then utilized to study the surface crystallinity. Individual peak intensities were extracted for different samples and different peaks.
Following this, ratios were calculated by dividing the peak height arising from trans conformations by the ones arising from gauche conformations. Two different ratios have been used to represent the surface crystallinity in order to see the reproducibility of the crystallinity approximation with different functionalities.
I1340/I1370 corresponds to the trans to gauche ratio of CH2 wagging vibrations and I1470/I1450 to the CH2 bending vibrations. The ratio of trans (crystalline/amorphous) to gauche conformers (amorphous) is expected to indirectly reflect the surface crystallinity of the PET samples. Figure 11c shows the change in the trans/gauche ratio of the DR1, DR2 and DR4 samples with respect to DR1 as the reference. The equation used to obtain these ratios is:
Change in peak ratio
=
𝑝𝑒𝑎𝑘 𝑟𝑎𝑡𝑖𝑜 𝑣𝑎𝑙𝑢𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑖𝑛𝑑𝑖𝑣𝑖𝑑𝑢𝑎𝑙 𝑠𝑎𝑚𝑝𝑙𝑒 𝑝𝑒𝑎𝑘 𝑟𝑎𝑡𝑖𝑜 𝑣𝑎𝑙𝑢𝑒 𝑜𝑓 𝐷𝑅1 𝑠𝑎𝑚𝑝𝑙𝑒The value obtained from this change is peak ratio corresponds directly to the increase in the value of surface crystallinity in the sample from DR1 sample. Hence, an increase in this fraction shows an increase in the surface crystallinity value for the sample under study.
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Figure 11. Different peaks from the FTIR-ATR spectra of DR1, DR2 and DR4 samples.
The plotting of the change in peak ratio for each sample led to a good trend regarding the surface crystallinity of the sample. A slight increase in the value of surface crystallinity is observed between DR1 and DR2 samples.
The change in value becomes steeper going from DR2 to DR4, which is a trend that was observed in thermal analysis as well since the crystallinity of DR4 increases very significantly as compared to DR1 and DR2 samples.
A very similar trend was observed upon plotting the values of crystallinity of the samples obtained by treating the data from the peak fitting process. The data obtained showed a small deviation (~5%) from the values obtained via Raman spectroscopy and DSC data.
4.1.3. Raman spectroscopy
To get deeper knowledge about the structural changes occurring in the core of the fibre, Raman spectroscopy was utilized. The Raman spectra obtained from analyzing DR1, DR2 and DR4 samples were normalized with respect to the peak at 1291 cm-1, corresponding to the C-C ring and C-O stretching, since this band shows a consistent intensity. The Raman spectra recorded for samples DR1 and DR4 are shown in Figure 12. The main peaks of interest have been highlighted in pink. Previously reported work has established that the ethylene glycol segment in the crystalline form of PET adopts a trans conformation, which gives rise to a peak at 1095 cm-1 [38]. The amorphous phase consists of glycol segments adopting gauche conformation, represented by peak at 1120cm-1, as well as trans conformations. This transition in conformation allows for the tracking of structural changes occurring throughout the diameter of the fibre. Other significantly visible changes occur
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at 860 cm-1, 1097 cm-1 and 1616 cm-1, while the width of the 1729 cm-1 peak also changes with increase in the draw ratio. The functionalities corresponding to these peaks demonstrating changes have been summarized in Table 10.
Figure 12. Raman spectra of DR1 and DR4 samples focused between 800cm-1 and 1800cm-1.
Table 10. Peak assignment for the Raman spectra of DR1, DR2, and DR4.
Peak position Assignment
860 cm-1 Ester C(O)-O stretching
1097 cm-1 Ester C(O)-O, ethylene glycol C-C stretching (trans)
1120 cm-1 Ring C-H in-plane bending, ester C(O)-O and ethylene glycol C-C stretching (gauche)
1616 cm-1 C-C/C=C benzene ring mode 8a
1729 cm-1 C=O stretching
In order to enable accurate extraction of data form the Raman spectra, peak fitting was carried out for specific peaks of DR1, DR2 and DR4 samples. This is carried out using a specially written python code. Peak fitting was also carried out to separate the coinciding peaks corresponding to trans and gauche conformers present in the structure of the fibres. Figure 13 presents the different peaks that were fit using one or two Pearson VII functions. Various shifts and changes in peak heights are observed upon looking at these plots on a closer scale. The width of the Raman band at 1729 cm-1 (C=O stretching) is significantly changing as we shift from a lower draw ratio to a higher draw ratio (from DR1 to DR4). Changes in the width of this Raman band have been associated with changes in crystallinity, density or conformations in the previously reported results [30, 38,]. Further, the height of the peak at 1616 cm-1 reduces upon increasing the draw ratio while the reverse is observed for the peak at 860 cm-1. The most interesting observations are made in the region between 1060 cm-1-1160 cm-1 were the peaks arising from the trans and gauche conformations of ethylene glycol segments
26
are situated. This peak fitting procedure was used to obtain data regarding the intensities, heights and widths of different peaks. This data was then used to extract the different peak ratios which would directly correspond to the crystallinity of PET fibres. The different ratios obtained were then used to prepare the radial maps of peak height ratios (Ipeak/I1120) with respect to the gauche peak at 1120 cm-1. These radial maps are shown in Figure 14.
Figure 13. Raman peak fits in the different regions for DR1, DR2 and DR4 samples.
The radial maps of peak ratios presented in Figure 14 exhibit interlocked network of high and low intensity regions, which correspond to the microscopically interlocked network of crystalline and amorphous regions.
Schematically, this effect can be observed from the mix of the colour ranges present throughout the map of the fibres. The changes occurring in the peak intensities can be attributed to changes in crystallinity, density or conformation of the functional group structure. The data obtained from the peak fitting shows that the average peak height ratios I860/I1120 and I1097/I1120 are increasing by a factor of two upon the fibre being drawn.
The change in I1097/I1120 occurs due to the change in conformation from gauche to trans of ethylene glycol segments. Peak height ratios I1616/I1120 and I1729/I1120 are however decreasing in intensity upon the effect of drawing. The peak at 1616 cm-1 has been assigned to the symmetric stretching of the 1,4-carbons of the benzene ring in the previously reported works [56] and hence it is a reflection of the molecular orientation of polymer chains [39, 40]. Since the incident light acting on the fibre is in the plane of the sample, decrease in intensity corresponds to the increase in the alignment of benzene rings in the axial direction of the fibre.
This gives rise to the higher molecular orientation for the highly drawn fibres [39, 41]. Significant changes have been observed in the region from 1040 cm-1 to 1160 cm-1. Two peaks are visible in this region which arise and disappear as an effect of fibre drawing. The Raman band near 1097 cm-1 increases in intensity upon being drawn due to the transition of gauche to trans conformation of the ethylene glycol bonds. This change is seen by the differences in the peak height ratio, I1097/I1120 in Figure 14. The peak height ratios I860/I1120 and I1097/I1120 are observed to be increasing upon drawing, with DR1 showing the least and DR4 showing the maximum value. The main reason for this change relates to the conformational changes from gauche to trans of ethylene glycol segments.
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Figure 14. Processed Raman data showing the changes in intensity ratios upon been drawn, a representative of changes in degree of crystallinity.
Raman spectroscopy has been used in the past to calculate percent crystallinity for various polymers.
However, it is important to be noted that changes in the trans and gauche intensity band may not arise only due to changes in the chain conformation and thus crystallinity, but can also be attributed to changes in chain packing and molecular chain orientations [42].
Further, to get the details regarding the changes in the crystallinity values across the map of the fibre, peak intensities were used to calculate the crystallinity map. This was done by using the same equation that has been reported previously in literature [43]:
𝜒𝑅𝑎𝑚𝑎𝑛(%) =𝐼1097− 𝐼1097𝑎
𝐼1117+ 𝐼1097 × 100
where I1097 and I1117 are the intensities of the peaks at 1097 cm-1 and 1117 cm-1 respectively, and I1097a is the intensity of the 1097 cm-1 vibration band of a 100% amorphous sample. As mentioned before already, PET structure comprises of the trans conformation even in 100% amorphous state and hence arises the need to subtract the intensity of amorphous peak from the crystalline peak. The value for I1097a was estimated to be
