Mar 29, 2023

Abstract Among the different fundamental aspects that govern the design and development of elongated multimaterial structures via the preform-to-fiber technique, material association methodologies hold a crucial role. They greatly impact the number, complexity and possible combinations of functions that can be integrated within single fibers, thus defining their applicability. In this work, a co-drawing strategy to produce monofilament microfibers from unique glass-polymer associations is investigated. In particular, the molten core-method (MCM) is applied to several amorphous and semi-crystalline thermoplastics for their integration within larger glass architectures. General conditions in which the MCM can be employed are established. It is demonstrated that the classical glass transition temperature compatibility requirements for glass-polymer associations can be overcome, and that other glass compositions than chalcogenides can be thermally stretched with thermoplastics, here oxide glasses are considered. Composite fibers with various geometries and compositional profiles are then presented to illustrate the versatility of the proposed methodology. Finally, investigations are focused on fibers produced from the association of poly ether ether ketone (PEEK) with tellurite and phosphate glasses. It is demonstrated that upon appropriate elongation conditions, the crystallization kinetics of PEEK can be controlled during the thermal stretching and crystallinities of the polymer as low as 9 mass. % are reached in the final fiber. It is believed such novel material associations as well as the ability to tailor material properties within fibers could inspire the development of a new class of hybrid elongated objects with unprecedented functionalities. Introduction Multimaterial fibers are considerably boosting the development of new generations of miniaturized devices and components with applications spanning all technological domains of interest (health, energy, electronics, environment, defense, telecommunications, etc.). The fabrication of such remarkable and undeniably useful objects relies on advanced material shaping processes, which are necessary to simultaneously embed multiple functionalities within fibers. Owing to strong contributions from the material science community, the range of materials that can be integrated into fiber assemblies 1 , as well as their possible combinations and the way they are arranged within the elongated structures have now immensely widened 2 , 3 , 4 , 5 . To achieve this, numerous preform preparation strategies and unconventional thermal drawing techniques were implemented, such as the stack-and-draw technique 6 , 7 , insertion methods 8 , 9 (derived from the rod-in-tube 10 , 11 ), extrusion 12 , additive manufacturing 13 , 14 , 15 as well as the direct wire insertion 16 , 17 , and so on. Among those popular multimaterial fiber preparation strategies, the molten core method (MCM) appears as one of the main attractive solutions for the design of intricate hybrid elongated structures 18 . The MCM relies on the following principle: a core material is inserted within an amorphous cladding to form a macroscopic preform and the assembly is subsequently stretched into thin continuous fibers using conventional fiber-drawing equipment, with the particularity that during the stretching procedure, the core material is in a molten state, while the amorphous cladding material is only softened. In other words, the cladding acts as a support to restrict the flow of the liquid core material. In the early development of the molten core technique 19 , the method referred to thermal elongation procedures during which chemical interactions within the core materials mix or between the molten core and the cladding would occur. This process would allow the production novel materials that could not be synthesized otherwise or could not be integrated into fibers through the conventional drawing process. The technique now designates, in a broader way, thermal elongation procedures during which a core material is drawn while being in the molten state 20 . To this date, the MCM has mainly been exploited for the in-fiber incorporation of inorganic, non-conventionally stretchable materials (glass compositions with high rare-earth concentrations, semi-conductors, metals, etc. ), but the integration of polymers into glass-based elongated structures using the same methodology has not been studied. It is proposed in this work to assess for the first time the fiber drawing ability of selected thermoplastics using the molten core method. First, the conditions in which the MCM can be applied to integrate polymeric materials within larger glass frames are discussed, and general thermal parameters are established. Following, experimental investigations are carried out and several fibered structures produced from unprecedented glass/polymer associations are examined. It is demonstrated that amorphous and semi-crystalline polymers can be co-drawn with low glass transition temperature oxide glasses while being in a low viscosity regime. This result constitutes an important breakthrough as glass/polymer associations are almost exclusively limited to the combination of chalcogenide glasses and high-performance amorphous thermoplastics. In a final section, investigations concerning the production of continuous poly ether ether ketone (PEEK) fibers are presented. Due to the wide MCM temperature range of PEEK, it is successfully elongated in combination with several oxide glass claddings of different compositions (tellurites and phosphates). The quenching effect caused by the thermal drawing was also investigated towards the control of the degree of crystallinity of PEEK fibers. Experimental section Glass synthesis and preform preparation All glasses considered in this work were fabricated using the standard melt-quenching procedure: TeO2 (Fox chemicals 99.9%), ZnO (Alfa Aesar 99.99%) Na2O (Alfa Aesar 99.99%), La2O3 (Alfa Aesar 99.99%) and Y2O3 (Alfa Aesar 99.99%) in the powder form were weighed in adequate ratios and put together inside a platinum crucible. The precursor mix was then heated up to 800–900 °C depending on the composition of the glass, and the temperature was maintained at this value for one hour. Following, the hot melt was poured into a preheated brass mold of chosen architecture so that glass parts with adequate shapes were produced (cylindrical, parallelepipedal, etc.). The glass was then annealed at Tg—10 °C for 5 h and its temperature was slowly (≈2 °C/min) ramped down to room temperature. The preforms were then fabricated using a combination of various methods (molding, mechanical drilling, stacking, insertion, etc.). Glass pieces are assembled together with various polymer parts (rods and films), which were bought from different suppliers: PEEK and PEK rods (Ensinger), PES films (GoodFellow), PLA, PETG, ABS, PC and PA 6–6 filaments (Ultimaker), see Table 1 for the meaning of the used abbreviations. Table 1 Glass transition temperature Tg, onset crystallization temperature Tc, melting temperature Tm and decomposition temperature Td of various amorphous and semi-crystalline thermoplastic polymers. Differential scanning calorimetry (DSC) DSC experiments were performed on a TA Instruments Q1000 system with standard aluminum pans. Temperature and energy calibrations of the system were realized using Indium (Tm = 156.6 °C) and sapphire. For all the PEEK samples (5–10 mg), the analysis method starts with a 5 min isothermal step at 23 °C followed by a heating ramp at 10 °C/min to 450 °C and a last 5 min isothermal step. The baseline was recorded in the same conditions and subtracted from each thermogram before analysis, using Thermal Analysis™ software ( www.tainstruments.com ). DSC experiments during the first heating lead to the determination of the glass transition temperature Tg, the melting temperature Tm, the melting enthalpy ΔHm, the cold crystallization temperature Tcc, the cold crystallization enthalpy ΔHcc, and the degree of crystallinity χc. The degree of crystallinity, χc, is calculated from Eq. (1) for samples showing a cold crystallization temperature. The value of the melting enthalpy for a 100% crystalline PEEK ΔHm100% considered is 130 J/g and taken in Blundell et al. work 21 . Equation (1): $$\chi c= \frac{\Delta Hm-\Delta Hcc}{{\Delta H}_{m}^{100\%}}$$ The values of ΔHm, ΔHcc and χc given in Table 4 are averaged from three different DSC measurements. The average standard deviation for χc is determined from those multiple measurements and is found to be 2%. Thermogravimetric Analysis (TGA) TGA experiments were performed on a TA Instruments Q600 system with standard platinum pans. The measurements were carried out by heating ≈15–30 mg samples from room temperature up to 700 °C at a 10 °C/min rate and under a 50 mL/min nitrogen gas flow. The decomposition temperature Td of the considered polymer corresponded to the temperature at which a 2% mass loss was reached on the TGA curve. Dynamic mechanical analysis (DMA) Viscosity curve of polymer samples was determined through unidirectional cylindrical compression model via a DMA system working in linear deformation mode 22 . DMA measurements were performed on few-millimeter-thick and 10 mm diameter cylindrical samples using a TA Instruments Q800 system in parallel plate configuration. The thickness of the sample was then monitored when heated at a 2 °C/min rate from Tg—20 °C up to an upper-temperature limit. This upper limit depends on the considered material and was the temperature at which full deformation of the sample occurred. The viscosity η is calculated from Eq. (2) 22 : $$\eta =\frac{2\pi F{l(t)}^{5}}{3V(2\pi {l(t)}^{3}+V)(1+\alpha \Delta T(t))\frac{dl}{dt}}$$ where l(t) (in m) was the sample thickness at a given time t, V (in m3) the sample volume, α (in K-1) the coefficient of thermal expansion, ΔT(t) (in K) the difference between room and the measurement temperature at a given time t and dl/dt (in m/s) the axial deformation rate. Note that the experiments were carried out with a constant loading force F = 2.0 N. Results and discussion Material considerations In this first section, we establish the general conditions in which the MCM can be applied for the integration of continuous thermoplastic material inclusions within larger glass structures. From a general standpoint, thermoplastic polymers can be defined as organic materials that soften upon heating and which are classified as either amorphous or semi-crystalline. Amorphous polymers have long been used for the development of optical fibers owing to their excellent thermo-viscous properties which are compatible with the thermal drawing process. They exhibit a slow viscosity change when heated above their glass transition temperature (Tg), which allows for flow control and thus the fabrication of milli-, micro-, or even nano-structured elongated arrangements 23 , 24 . Semi-crystalline materials on the other hand, do not soften when heated above Tg due to their structure being a mix of rigid crystalline phases and amorphous domains, and are therefore not suitable for fiber-drawing. PEEK is for instance known for having crystalline phases with lamellar (10 nm) or spherulitic morphologies (1-10 µm) dispersed within an amorphous matrix 25 . Upon heating, such semi-crystalline polymers rapidly transform into a low-viscosity liquid when reaching temperatures above their melting point Tm. The rheological behaviors of the two types of thermoplastic polymers mentioned here are illustrated on Fig. 1 a, where viscosity curves of amorphous PES (orange symbols) and semi-crystalline PEK (black symbols) are plotted as function of temperature. Note that PEK has a considerably sharper viscosity variation as compared to PES, with ηPEK decreasing from 108 to 105 Pa.s over a ≈10 °C temperature range while this viscosity variation occurs over a ≈100 °C temperature range for ηPES. That kind of rheological behavior prevents semi-crystalline polymers from being stretched into fibers through the conventional drawing method. Figure 1 (a) Viscosity curves of amorphous polyether sulfone (PES) (orange symbols) and semi-crystalline polyether ketone (PEK) (black symbols). Viscosity ranges for the conventional fiber-drawing method (106 to 104 Pa s) and the molten core technique (η < 104 Pa.s) are displayed on the graph. (b) TGA curves of various amorphous or semi-crystalline thermoplastic polymers (see Table 1 for the meaning of the used abbreviations). The molten core technique can be applied to polymers when their viscosity reaches values below 104 Pa.s, which is considered as the lower viscosity limit for regular thermal drawing 26 . For semi-crystalline organic materials, this corresponds to temperatures above Tm. Based on the viscosity curve of PES shown on Fig. 1 a, the MCM lower temperature limit is 320 °C, which corresponds to approximately Tg + 100 °C. This empirical value will be referred to in the remainder of the manuscript as a first approximation of the lower MCM temperature limit for amorphous polymers. On the other hand, the upper stretching temperature limit of both amorphous and semi-crystalline thermoplastics in the molten state can be defined as the decomposition temperature Td, i.e. the temperature at which a 2% mass loss is measured through thermogravimetric analysis (TGA) at a 10 °C/min heating rate. TGA curves of various thermoplastics are plotted on Fig. 1 b. The corresponding decomposition temperatures are recapped in Table 1 along with other characteristic temperatures. In summary, the temperature range in which the MCM can be applied to thermoplastics is specified here as Tm—Td and Tg + 100 °C—Td for semi-crystalline and amorphous polymers respectively. This temperature range is reported on Fig. 2 for various polymers and compared to the thermal-drawing temperature domains of several glass families. Figure 2 Drawing temperature ranges of various amorphous and semi-crystalline thermoplastics and glass materials using the conventional fiber-drawing method (open rectangle) or the molten core (shaded rectangle) method (see Table 1 for polymer acronyms). Drawing temperature ranges of known glass compositions (SiO2, As2S3, NaPO3 and ZBLAN) and glasses investigated in the present work (TZN80, TZL65 and PNaKAl, see Table 3 for compositions) are shown on this figure (filled rectangle). Conventional drawing temperature domains of different classes of materials (amorphous polymers, low, intermediate and high Tg glasses) are also shown on the bottom part of the graph. It is interesting to note that the drawing temperature domains of amorphous polymers and low Tg oxide glasses do not overlap. From Fig. 2 , it is clear that the MCM applied to thermoplastics not only expands the range of polymers that can be integrated into glass fibers (extension to semi-crystalline polymers), but also the range of possible glass-polymer combinations. With few exceptions 27 , glass-polymer associations have long remained limited to the embedding of chalcogenide glasses within high-performance amorphous thermoplastic claddings such as PES or PEI 23 , 28 . Semi-crystalline polymers on the other hand, have mainly been stretched into fibers in association with amorphous thermoplastics 29 , 30 but never, to the best of our knowledge, with glasses. The methodology described here opens new opportunities for the design of intricate multimaterial fibers based on unusual materials combinations. It indeed bridges the gap that exists between the conventional drawing temperature ranges of amorphous polymers and low Tg oxide glasses (see Fig. 2 ). A panel of several glass-polymer associations for fiber fabrication is presented in the following sections to illustrate the potential of the presently described method. First results In this section, several materials combinations are explored to produce elongated glass/polymer structures. The preform fabrication methodology relies on conventional preform fabrication strategies, namely the insertion technique (insertion in a tube manufactured through mechanical drilling or molding procedures), stack-and-draw, etc. Description of the general preform fabrication process and subsequent thermal drawing is described in Fig. 3 a: simple glass pieces (plates, tubes, etc.) are combined with polymer rods, canes (cylindric, rectangular, etc.) and films into a macroscopic preform with arbitrary geometry which is then elongated into tens-of-meters of continuous fiber. Figure  3 b–j illustrates the various glass/polymer associations that are investigated in this work. First, simple and small-size cylindrical inclusions of thermoplastic are integrated within larger glass structures. Information about the different glass/polymer combinations that are tested is summarized in Table 2 . Figure 3 (a) Description of the molten core method applied to glass-polymer associations (image created using blender software version 2.92.0, www.blender.org/ ). Cross-sectional view of (b) a TZL65 glass cladding / PEK core fiber and a (c) TZL65 glass cladding / PEEK core fiber. (d) and (e) SEM images of a TZL65 glass cladding / PEK core fiber with the thin PEK core poking out of the glass cladding. (f) Picture of the neckdown region and canes from the failed stretching of a TZN60/PLA composite preform. (g) Longitudinal and (h) cross-sectional view of a rectangular TZN60 glass fiber with a central continuous Nylon inclusion. (i) and (j) SEM images (× 300 and × 10,000 respectively) of a TZN75 glass fiber integrating a thin PES structure. Figure  3 b,c show a TZL65 glass fiber (glass compositions 65 TeO2–30 ZnO–5 La2O3 mol %) with PEK and PEEK cores respectively. Remarkably, the continuous organic inclusion runs all along the fiber length, as illustrated on Fig. 3 d,e, where a thin continuous PEK filament is seen pocking out of a larger glass cladding. A similar successful integration of a Nylon wire within a rectangular TZN60 glass cladding (glass compositions 60 TeO2–20 ZnO–20 Na2O mol %) is depicted on Fig. 3 g,h. The co-drawing of the selected materials is possible because, as discussed in the previous section, the drawing temperature of the cladding glass lies between the melting and decomposition temperatures of the considered semi-crystalline polymers. When this criterion is not respected, the organic structure risks severe degradation, as seen in Fig. 3 f, which describes the neckdown region and canes resulting from the thermal stretching of a PLA rod (8 mm in diameter) within a TZN60 glass tube. The drawing temperature range of the vitreous cladding material exceeds the decomposition temperature of PLA (350–370 °C against 309 °C respectively), which leads to the deterioration of the thermoplastic that turns out completely carbonized after the drawing process. Indeed, those two materials do not appear compatible for the MCM from the diagram given in Fig. 2 . Finally, a more complex sandwich-like fiber architecture is produced from the combination of Polyether sulfone and a TZN75 tellurite glass (glass composition 75 TeO2–20 ZnO–5 Na2O mol %). The preform is prepared from two rectangular glass plates and a 50 µm thick PES film using a modified stack-and-draw method 31 . SEM images of the fabricated fiber are shown in Fig. 3 i,j. The thin PES structure is preserved after thermal stretching, and more remarkably its thickness is submicronic (≈0.7 µm, see Fig. 3 j) and stays well uniform on the whole fiber section. No breakup due to capillary instability is observed on the fabricated elongated structures, despite the polymer ribbon meeting the requirements for it to potentially occur (namely low viscosity and submicronic thickness) 32 . Note that the two materials combined in this architecture are both amorphous but have a significant glass transition temperature difference of ≈83 °C (Tg = 225 °C and Tg = 308 °C for PES and TZN75 glass respectively), which means they possess very distinct conventional drawing temperature ranges (285–340 °C for PES and 390–410 °C for the TZN75 glass). As the TZN75 parts represent ≈99.5% of the volume of the composite preform considered here, its thermal stretching is carried out at the drawing temperature of the glass, which implies that PES is in the molten core viscosity domain during the procedure (i.e. η < 104 Pa.s, see Fig. 1 a). It shows that the classical thermal limitation related to Tg compatibility can be overcome when considering glass/polymer associations for fiber fabrication. We have proposed highly unconventional glass/polymer associations to produce elongated architectures. Given the nature of the combined materials, fibers composed of spatial domains with important chemical reactivity contrasts (towards specific acids or solvents) could be manufactured. This aspect may be useful for further functionalization of the fabricated objects through targeted post-drawing processing procedures (redox reactions, dissolutions, selective etching, spatial segregation, etc.). The authors have for instance demonstrated that such composite assemblies enable the fabrication, through selective etching of the glass cladding, of quasi-exposed core fibers which have potential for the evanescent wave probing of liquids 33 . Also, the integration of thermoplastics could help bring new functionalities (mechanical, electrical, optical, etc.) within glass fiber structures especially if specialty composite polymers (with various loads: metals, ceramics, glass fibers, nanoparticles, conductive loads, fluorescent dyes, etc.) are considered. A wide variety of elongated objects with unprecedented functionalities could in fact be produced from the methodology proposed in this work. Glass cladding / PEEK core fibers Fiber fabrication In this section, we focus on the incorporation of PEEK within larger elongated glass structures using the molten core method. PEEK is a very popular material among thermoplastics, mainly because it can sustain relatively elevated temperatures, has excellent mechanical and chemical robustness as well as good biocompatibility 34 . Fiber technology could benefit greatly from the unique properties of such thermoplastics. PEEK coatings for optical fiber reinforcement are already commercially available and enable reliable operation in harsh environments. Typically, PEEK coatings appear useful for applications in aerospace or energy industries. However, due to its rheological properties, PEEK cannot be stretched into fibers using the conventional drawing technique, as described in Fig. 4 a–d. In order to be shaped, this semi-crystalline polymer has to be heated above its melting temperature. Beyond this temperature, the viscosity abruptly falls (see Fig. 1 a which illustrates the evolution of the viscosity of semi-crystalline PEK) and the material flow cannot be controlled. This point considerably hampers the production of fibers with uniform and preserved cross-sectional ratio. Figure  4 b shows a longitudinal view of a PEEK fiber fabricated using the conventional thermal drawing procedure and which has a non-constant diameter. The formation of beadings along the fiber can be observed (see Fig. 4 c) due to flow discontinuity of the polymer. In order to control PEEK flow during thermal drawing, it must be embedded within an amorphous cladding (here glass) as in the molten core method. PEEK is selected because it has a wide MCM temperature range (from Tm-PEEK ≈340 °C to Td-PEEK = 616 °C) and can be combined with a large variety of glasses (chalcogenides, fluorides, tellurites, phosphates, etc. ), as long as their drawing temperate is within the MCM temperature range of PEEK (see Fig. 2 ). Figure 4 (a) Description of the fabrication process of a single-material PEEK fiber through the conventional drawing process (image created using blender software version 2.92.0, www.blender.org/ ). (b) Picture of a ≈5 cm long single-material PEEK fiber with a non-constant diameter. Portions of fiber with (c) or without (d) diameter irregularities (beadings). (e) Description of the fabrication process of a composite Tellurite/PEEK fiber through the molten core method (image created using blender software version 2.92.0, www.blender.org/ ). (f) and (h) cross-sectional view of the Tellurite/PEEK composite fibers with various core/cladding ratios. (i) Fiber with a bubble defect. (g) Longitudinal view of the composite fiber with the central PEEK core poking out at the fiber tip. (j) SEM image (× 205) of a roughly cut Tellurite/PEEK fiber. (k) ≈60 m of tellurite/PEEK fiber stored on a spool. (l) Picture of a fractured segment of the composite fiber. Here, we selected a tellurite glass cladding to apply the MCM to PEEK. First, a glass tube closed at the bottom, with 16 mm and 10 mm of outer and inner diameter respectively is fabricated using a simple casting method. The length of the fabricated glass tube is approximately 60 mm. Then, a 10 mm diameter and 60 mm long peek rod is inserted inside the tube and the assembly is subsequently thermally drawn into meters of composite fibers. A description of the process is depicted in Fig. 4 e. Various views of the resulting fibers are shown on Fig. 4 h–j. Structures with diverse glass-to-PEEK ratios can be obtained by changing the dimensions of the glass tube and the PEEK rod (e.g. using a glass tube with respectively 9 mm and 1.1 mm external and internal diameters, and 1.1 mm PEEK cane), as seen in Fig. 4 f,g. Remarkably, the structure is well uniform, as highlighted by the length of fiber produced and depicted on Fig. 4 k. The PEEK inclusion also appears continuous (see Fig. 4 l) over the whole fiber length. The glass cladding, if drawn at the proper viscosity, acts as a support that restricts the flow of polymer and helps control the uniformity of the organic structure and the overall fiber cross-section. Degree of crystallinity of PEEK We now investigate the influence of the molten core method on the degree of crystallinity of PEEK when elongated within a vitreous material. During the thermal drawing, the polymer is molten and forms a liquid bath within the glass cladding just above the neckdown region. As the stretching is carried out, the molten PEEK flows through the neckdown region with the glass after which it rapidly solidifies to form the composite fiber. Because of the micrometric dimensions of the final fibered objects, both the polymer and the glass are very quickly cooled down, in a matter of few seconds, from the drawing temperature to room temperature. This treatment is equivalent to a quenching process. It is widely admitted that the crystallization kinetics of a melted polymer during cooling is directly dependent on the rate of temperature change 35 , 36 . For this reason, it is decisive to investigate the degree of crystallinity of PEEK in fibers fabricated using the molten core method, especially when several Physico-chemical properties are directly correlated to this parameter (density, tensile strength, heat resistance, etc.). A PEEK rod is co-drawn with three different tellurite or phosphate glass claddings which have either a low, intermediate or high glass transition temperature. Compositions of the glasses used for the MCM are recapped in Table 3 along with their glass transition temperatures and thermal drawing temperature ranges. Fibers with a comparable PEEK/glass diameter ratio of ≈0.6 are produced for this experiment with all three claddings using the methodology described in the previous section. MCM drawings are performed in similar conditions (preform feed of 0.5 mm/min, draw speed ranging from ≈1 to 5 m/min, depending on the fiber diameter). Table 4 presents the thermal transitions (Tg, Tc, Tm), enthalpy of cold crystallization (ΔHcc), enthalpy of melting (ΔHm) and degree of crystallinity (χc) for the starting commercial rod of PEEK, for a PEEK fiber produced using the conventional thermal drawing process (without glass cladding) and for PEEK core fibers produced using the MCM with either a low, intermediate or high Tg glass (noted MCMF-L, MCMF-I, and MCMF-H respectively). The degree of crystallinity is averaged from the results of three measurements for all samples (see “ Experimental ” section). Results for PEEK fibers of different diameters are presented in Table 4 . The values are extracted from the DSC curves such as the ones plotted in Fig. 5 d. It is worth mentioning that for PEEK samples coming from the MCM, the glass cladding is removed prior to the DSC measurement (as described in Fig. 5 a). First, the fiber is immersed in concentrated hydrochloric acid (6 mol L−1) for a few minutes to dissolve the glass. Then, the PEEK core is retrieved, thoroughly rinsed in deionized water and dried at 80 °C for 1 h in a vacuum oven. Figure  5 b,c show pictures of bare PEEK samples after the glass cladding has been removed. The starting material, i.e. the PEEK commercial rod as well as the fibers obtained when the polymer is elongated alone, show high degrees of crystallinity since χc values reach 30%. Similar values are measured for samples with larger diameters, i.e. for portions where beadings form along conventionally drawn PEEK fibers (noted PEEK fiber—beadings in Table 4 ). For these samples, no cold crystallization is observed during the first heating (see DSC curves in Fig. 5 d), which means that the transformation process (rod fabrication and drawing) applied to the PEEK induces a sufficiently low cooling rate to allow for the crystallization of the samples to take place. Moreover, the drawing process itself induces a regular alignment of the polymeric chains in the direction of the flow 37 leading to a highly packed and thus crystallized material. However, when the molten core method is applied to PEEK/glass systems, a strong decrease of the degree of crystallinity of PEEK is observed, with χc values decreasing down to 9 mass. % for some samples. As the solid-state organization of this polymer is known to be very sensitive to the cooling rate 35 , a high cooling rate applied to the molten PEEK results in a near-quenched material showing a low degree of crystallinity. Table 3 Composition, glass transition temperature Tg and thermal drawing temperature range Tdraw of the different glasses used as claddings for the MCM applied to PEEK.