1. Introduction
The constantly growing demand in carbon fiber reinforced plastics (CFRPs) in performance-demanding fields such as (aero-)space engineering, automotive, construction, and sport equipment highlights the relative importance of further improving and optimizing the production process and surface chemistry of carbon fibers (CF) in order to fabricate cost- and energy-efficient composites [
1,
2,
3]. This can be better achieved by modifications of the stabilization process [
4,
5,
6,
7,
8,
9,
10], since carbonization is a straightforward process that cannot be essentially altered and that investigation on novel CF precursors is time a consuming procedure. Polyacrylonitrile (PAN)-based CFs are more efficiently manufactured (due to their high carbon yield) and demonstrate a structure with less defects and voids, affording CF with high tensile strength in comparison to CFs derived from pitch, Rayon, or renewable precursors [
1,
10,
11,
12,
13,
14,
15]. Given that PAN-based CFs represent about 80–90% of the world production, there is a strong incentive for improving the process design, so that it might be further optimized [
9,
14,
16].
Stabilization is considered as the most crucial step and obtaining a proper structure is critical for achieving high performance CFs. This oxidation step, leads to an infusible and inflammable ladder polymer structure, suitable for carbonization [
11,
14,
17,
18,
19,
20]. Regarding the thermal treatment, processability of PAN fibers is enhanced at temperatures exceeding glass transition temperature (T
g), commonly above 170 °C [
21]. In this temperature region the PAN precursor is partially plasticized and this is the point where an improvement in fiber orientation could be achieved, while tension assists the process and leads to enhanced molecular alignment in fibers [
22,
23,
24]. Molecular structure is affected by several reaction pathways that transform the polymer backbone of fused heteroaromatic structures; furthermore, the presence of heteroatoms such as residual hydrogen, nitrogen and oxygen can lead to crosslinking, and thus to higher carbon yields and an improved structure upon carbonization [
6,
11,
25,
26,
27,
28,
29].
During the stabilization process, physical and chemical transformations occur that can be observed by the change in fiber length. The latter is dominated by three phenomena, namely: entropic shrinkage, creep, and chemical shrinkage [
22,
30,
31]. The utilization of length change for the in-depth investigation of the stabilization process makes this approach based on the deconvolution of the corresponding phenomena that take place during the process. Entropic shrinkage (ES) or recovery is a physical transformation that occurs at early stabilization stages upon initiation of stabilization reactions. It is attributed to relaxation of highly orientated PAN macromolecules, due to the spinning and post-spinning treatment and the reduction of crystalline regions during thermal treatment. It was found that maximum Entropic Shrinkage (ES
max, i.e., the reduction of the PAN fibers length during the primal stage of oxidative stabilization in the limiting case where no stress is applied), is a physical constant that depends on the structure of the PAN homo/co-polymer fiber [
1]. This phenomenon is derived by the helical form of PAN macromolecule and the interaction in an intermolecular level, which is attributed to repulsive forces of the adjacent carbon-nitrile tribble bond dipoles [
24]. Another origin connected to fiber shrinkage is the relaxation effect induced by the heat flux during thermal processing; the pre-existing stretching-induced strains amongst macromolecules by the fiber manufacturing are rearranged to a random and lower energy configuration [
1,
22,
32,
33]. In contrast to entropic shrinkage, creep is a heat activated phenomenon which is dependent on temperature and applied stress. These parameters can be adjusted in order to prevent shrinkage, retain post-spinning treatment orientation, and further orientate PAN molecules [
22,
23,
28,
32,
34]. However, fiber stretching during oxidative stabilization should be applied sparingly, as it is possible to disturb the preferred orientation and hinder the cyclization propagation due to extensive debonding, and thus lead to reduction of mechanical properties [
1,
13,
24,
35]. Chemically-induced shrinkage is derived by exothermic chemical reactions, which lead to ladder polymer structure, and it was shown that chemically-induced shrinkage could be used to monitor the nitrile cyclization yield [
1,
24,
33]. The kinetics and extend of length change depends on factors such as the atmosphere, temperature, the applied stress, stretch given to precursor fiber during spinning process, as well as the heating rate [
1,
13,
24,
36].
Generally, the main chemical reactions that occur during stabilization are dehydrogenation, cyclization, and oxidation. Their initiation depends on the treatment temperature, which also determines the relative effect and the relative rate of each reaction at the specific treatment stage [
7,
11,
13,
14,
24,
29,
32,
37]. Dehydrogenation reactions are considered to precede cyclization, while the oxidation takes place in the whole temperature region of stabilization. These reactions are responsible for the gradual colour change of the precursor fiber, starting from white to yellow, to brown, and further spectrum to obtain black colour by the end of dehydrogenation [
24,
38]. The colour change is assumed to be result of polyene structures produced during stabilization of PAN, while it is also claimed that black colour is attributed to the absorption of the visible spectrum wavelengths by the condensed ring structure. This structure is formed by cyclization-induced carbon-nitrogen double bonds (
Figure 1), known as ladder structure. Heating of PAN fibers in an air atmosphere in the low stabilization temperature ranges initiates dehydrogenation reaction, and double bonds are introduced in the chain backbone, which improve thermal stability of the chain [
7,
29].
Cyclization is considered the dominant stabilization reaction, which is the main occurring reaction when temperature is raised in the region from 180 to 250 °C. The end products are various imine structures among PAN macromolecules though oligomerization of nitrile groups, via linkage between nitrogen and the carbon atom of the succeeding nitrile group of the chain [
7,
11]. Cyclization is initiated at several activation spots of PAN macromolecule during thermal processing, and evolves intil its growth is terminated by reaching another conjugated or cyclized unit [
24]. Numerous reaction paths have been introduced in the literature, based on either self-initiation (mainly in the case of copolymers with oxygen-containing comonomers that act as initiation points) or on external initiation (from the attack of the oxygen on the polymer backbone) [
24,
39]. Initiation occurs via a radical polymerization mechanism in regards to the homopolymer PAN and through an ionic mechanism, due to the presence of polar functional groups in PAN copolymers, which acts as electron donners, however, the reaction mechanism (or even the sequence of the reactions) does not really affect the structure of stabilized PAN fibers [
6,
13,
24]. However, it is widely accepted that chemical shrinkage, is induced exclusively by the nitrile cyclization reactions [
19,
40,
41].
Cyclization is followed by higher temperature oxidation, in order to derive the final pyridine structure. Generally, oxidation has a twofold effect on the stabilization process: oxygen can initiate the formation of active cyclization centers above 240 °C as oxygen diffusion is the dominating pathway for the transformation of the PAN fibers [
13,
24]. It has also been stated that at elevated temperatures, oxidation reaction occur at a higher rate than cyclization, even though between 180 °C and 240 °C cyclization is the dominating reaction [
1,
7]. However, it has been reported that further aromatization and intermolecular crosslinking reactions occur in the fiber while performing oxidative stabilization at 300–400 °C (
Figure 2). This leads to formation of highly aligned and compact structural units in the stabilized fiber (
Figure 3) [
24,
25]. Molecular chains in the fiber undergo stereochemical transformations, and reinforcement, to form pockets of small-sized basic structural units that maintain a crystalline distance of 6.8 Å [
24]. As the stabilization temperature increases, the polymerization kinetics are accelerated.
In most case studies, the object of stabilization investigation is the description of reaction mechanisms and the effect on structural changes; using microscopy, spectroscopy, and thermal analysis kinetic studies on cyclization and oxidation are performed, and progress of the reactions (e.g., cyclization) is estimated [
19,
30,
31,
40,
42]. While this approach seems time consuming, industrial scale stabilization can be better optimized through a combination of time-consuming experimentation, together with the application of empirical laws that relate temperature, tension, and time with the mechanical performance of the synthesized CFs [
29].
The aim of this investigation is to develop a descriptive model that will correlate the length change during stabilization of PAN fibers in the variation range of stabilization main parameters (temperature, time, and applied stress) and correlate it with the structural changes. Such a model could use the length change during the process as an effective control parameter. The foundation of the model is to monitor the main stabilization reactions by using macroscopic observations on the change in fiber length, as well as to establish the relation between length variation and stabilization parameters for each one of these phenomena. Since the correlation between cyclization and chemical shrinkage has been well established, the latter can be used as a measure to quantify the estimation of the stabilization progress, given that cyclization is considered as the most crucial reaction during the formation of the ladder polymer. By using the extracted model, it is expected that the number of the necessary experiments for optimizing the process will be minimized. Fourier Transform Infrared Spectroscopy (FTIR) was used to monitor the bonds formation and Differential Scanning Calorimetry (DSC) to measure the energy evolution during thermal processing, in order to quantify cyclization yield and compare the results to the model predictions. Further characterization was also performed to identify structural transformations during stabilization, such as optical microscopy in order to measure the deviation from circularity of fiber cross section.