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Temperature dependent network properties of amorphous PCT during tensile stretching
Journal Pre-proof Temperature dependent network properties of amorphous PCT during tensile stretching Zhongshuo Miao, Yongfeng Men PII:
S0032-3861(19)31044-4
DOI:
https://doi.org/10.1016/j.polymer.2019.122038
Reference:
JPOL 122038
To appear in:
Polymer
Received Date: 4 October 2019 Revised Date:
22 November 2019
Accepted Date: 28 November 2019
Please cite this article as: Miao Z, Men Y, Temperature dependent network properties of amorphous PCT during tensile stretching, Polymer (2019), doi: https://doi.org/10.1016/j.polymer.2019.122038. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Graphic Abstract
Temperature Dependent Network Properties of Amorphous PCT during Tensile Stretching Zhongshuo Miao, Yongfeng Men* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, University of Chinese Academy of Sciences, Renmin Street 5625, 130022 Changchun, P.R. China
ABSTRACT: Tensile deformation behavior of amorphous Poly (1,4-cyclohexylenedimethylene Terephthalate) (PCT) at 25 oC to 108 oC was investigated by means of in situ synchrotron wide-angle X-ray scattering (WAXS). WAXS patterns were processed in a way that the scatterings from isotropic and anisotropic structures were separated. It turned out that the influences of temperature during deformation can be divided into three zones: below 80 oC, only molecular chains oriented; between 80 oC to 90 oC, very few imperfect crystals were induced by strain; above glass transition temperature (92 oC), the triclinic PCT crystalline structure began to form during deformation process. It was found that the Gaussian model of Haward and Thackray described the stress-strain behavior of the tensile stretched amorphous PCT very well. However, when being stretched above 80 oC, the system showed two characteristic network moduli because of strain-induced crystallization. The onset of strain-hardening occurred immediately after the formation of unoriented crystalllites which served as physical cross-linking points to reinforce the material.
Key Words: network; tensile stretching; semi-crystalline polymers; X-ray diffraction; stress-strain curve
INTRODUCTION Compared to other thermoplastics, such as polyethylene terephthalate and Polybutylene terephthalate, Poly(1,4-cyclohexanedimethylene terephthalate) (PCT) has excellent thermal, hydrolysis stability and electrical properties[1-3]. The application of PCT has been expanded to films and fibers due to its extremely low oligomer content and good chemical resistance[3,4]. Despite its high application value, only few researches concerned the physical properties of PCT, like crystallization and mechanical properties[5,6]. Considering the crystallization behavior,
random
copolymers
terephthalate)(P(ET-co-CT)), (P(BT-co-CT)),
such
as
poly(ethylene
terephthalate-co-1,4-cyclohexylenedimethylene
poly(butyleneterephthalate-co-1,4-
poly(1,4-cyclohexylene
dimethylene
cyclohexanedimethylene
terephthalate)
terephthalate-co-hex-amethylene
terephthalate)
(P(CT-co-HT)), poly(trimethylene terephthalate-co-1,4-cyclohexylene dimethylene terephthalate) (PTCT) were found to exhibit composition dependent eutectic melting and isodimorphic crystallization behavior[7-13]. It was also discovered that PCT showed mechanically more ductile property than PET, which has its origin of the existence of cooperative secondary relaxation process because of the conformation transition of cyclohexylene rings from boat to chair. In many cases, PCT may end up with a quasi-amorphous structure after production due to its high glass transition temperature (Tg) being much higher than room temperature. Further mechanical treatment may lead to a continuous crystallization so that change in final properties. Therefore, strain-induced crystallization is of particular importance in the research of physical properties of PCT. The first strain-induced crystallization phenomenon in polymers may be dated back to 1925 when Katz investigated natural rubber (NR)[14]. Mechanical properties of NR can be strongly affected by the strain-induced crystallization, especially the crack propagation resistance[15,16.17] and fatigue performance[18]. Based on the development of thermal measurements and in situ wide-angle X-ray scattering (WAXS) technologies, many
literatures have reported the explanation of the mechanism of strain-induced crystallization, and provided important knowledge on crystallinity and size and orientation of the crystallites[14,19,20]. Flory[21] and Toki[22] considered that the crystallites induced by the strain were extended chain crystals which led to a reduction of the stress. Consequent crystal growth was concluded to be parallel to the drawing direction. The finite extensibility of the polymer chain network was thought to be the reason of the increase in crystallinity and stress. Tosaka et al.[23] reported network chain density independency of the onset strain of deformation induced crystallization. Clearly, stretched molecular chains seemed to act as nuclei while the crystal growth must be contributed by the involvement of the surrounding chains. Deformation caused structural variations in amorphous PET below and above Tg have been investigated extensively. Blundell et al.[24] observed the appearance of a structure of smectic A during fast tensile deformation of PET. The smectic A structure was proposed to be a precursor for further crystallization because the triclinic crystalline diffraction peak appeared simultaneously with the disappearance of smectic A one. Moreover, a smectic C structure was reported to appear during tensile stretching of amorphous PET at 50 °C below Tg by Ran et al.[25]. The mesophase was found to develop immediately after necking. The mesophase presented a sharp meridional peak 001′with a d spacing of 0.032 nm. This d spacing is smaller than the length of a monomer in typical triclinic unit cell. It was then assumed that the chain segments in mesophase gave a tilted smectic C structure. Stress-strain behavior of amorphous PET above Tg has been also connected to the structural variation. It was shown that necking induced the formation of the mesophase immediately. Afterwards, a three-dimensional network composed of mesomorphic chains and imperfect crystals was formed in the strain-hardening regime followed by the formation of mesophase and stable crystals after strain-hardening[26].
Up to now, knowledge about
strain-induced structural evolution in amorphous PCT remains incomplete. In this work, we studied the structural evolution of amorphous PCT during tensile deformation at different temperatures by in situ synchrotron wide-angle X-ray scattering (WAXS) techniques. Mechanical response of the PCT samples at different temperatures has been successfully linked to the formation of crystallites during stretching.
EXPERIMENTAL SECTION Materials and Samples. The PCT polyester was an industrial material，trade name SkyPURA0302, produced by SK chemicals. It has a number-average (Mn) and weight-average molecule weight (Mw) of 22.5 and 48.4 kg/mol, respectively. A film with a 0.5 mm thickness was obtained by compression-molding the material at 300 °C. Thermal history of the material was removed by keeping the film at 300 °C for 5 min. The thus pressed film was then quenched into ice-water to obtain an amorphous PCT film. The “dog bone” tensile specimens with sizes of 25 × 8 × 0.5 mm3 were obtained using a punch. The samples were tensile stretched with a TST 350 tensile testing machine (Linkam, U.K.). To obtain exact strain values at the X-ray beam position, we took optical photo images of the sample during stretching. Hencky strain εH was used as a quantity of the extension. εH is defined as follows: εH = 2 ln(b0/b) where b0 and b are the widths of undeformed and deformed area, respectively.
Characterization. Synchrotron WAXS measurements were conducted at the beamline 1W2A, BSRF, Beijing. The beamline was running under an X-ray wavelength of 0.154 nm. To record WAXS patterns, we put a MAR CCD detector 83.6 mm away from the sample position. The detector has a resolution of 2048 × 2048 under pixel size of 79 µm. A constant crosshead speed of 10 µm/s at deformation temperatures of 25 to 110 °C was utilized. The primary X-ray beam was illuminated in the middle of the sample which was horizontally placed. The WAXS patterns were recorded during deformation process, and exposure time was 23s for every pattern.
In order to study the orientational behavior of crystals and amorphous phase, we implemented the “halo method” suggested by Hsiao et al. [27,28] for the analysis of 2D WAXS patterns. The distortion effect caused by flat-plate detector on the WAXS patterns was corrected using Fraser method[29]. In the “halo method”, azimuthal scans at different q values were collected to yield scattering intensity distributions. In each azimuthal scattering intensity distribution curve, the minimum intensity value was assigned to the isotropic scattering contribution at this particular q position. By subtracting thus obtained isotropic scattering intensity distribution from the complete WAXS pattern, we obtained anisotropic scattering pattern. In both isotropic part and anisotropic part, there are contributions from crystalline and amorphous phases. The mass fractions of crystals and amorphous phase both oriented and unoriented were quantitative evaluated using a peak fitting method[30]. Differential scanning calorimeter (DSC) measurements were carried out with a DSC1 Stare System (Mettler Toledo Swiss). The set-up was calibrated by using indium as a standard for temperature and melting enthalpy. A heating rate of 10 K/min was used during the measurements for PCT samples.
RESULTS AND DISCUSSIONS 1. Deformation Behaviors and Gaussian Model Selected true stress−strain curves of amorphous PCT polyester at different temperatures from 25 oC to 108 oC are summarized in Figure 1. A complete set of such curves at more deformation temperatures can be found in Figure S1 in supporting information document. First of all, the elastic deformation occurred only within a very low strain range of less than 0.05 at all temperatures. At stretching temperature below glass transition (90 oC), a strain softening with a slightly decrease of true stress took place after the yield point whereas no distinct yielding was observed above 90 oC. Before strain-hardening, the true stress dramatic decreases with the increase of temperature from 25 to 90 oC.
After strain-hardening, a rather similar slop was observed in the stress-strain curves at each
temperature. These behaviors reveal that the deformation properties of amorphous PCT are strongly influenced by the stretching temperature. Because the molecular chain is frozen below Tg, the movement of the molecular chain segments is limited when being stretched at temperatures below Tg. One would not expect too much ordering of the chain segments into crystalline structure. Strain-induced crystallization, however, can take place when the material is stretched at temperatures above Tg. This behavior is reflected also onto the macroscopic stress-strain dependency. The stress-strain curves for samples stretched below Tg show a rather strong increase in stress before yielding whereas they exhibit much larger strain softening behavior after yielding. Eventually, the stress increases significantly at large strains indicating the occurrence of strain-induced crystallization when the samples were stretched above Tg.
Figure 1. True stress−strain curves of stretched PCT samples at different temperature.
In order to investigate network properties of PCT samples during stretching at different temperatures, a model developed by Haward and Thackray [31,32] can be applied. The model was previously developed to describe tensile stress-strain behavior of glassy polymers and has also been successfully applied to semi-crystalline polymers. The model is built upon three elements with an Eyring dashpot being in parallel with a Gaussian rubbery spring which are then in series with a Hookean spring. The model implies that at large strain regimes the deformation of semicrystalline polymers should be determined by the stretching of the Gaussian rubbery spring network. The property of this Gaussian spring network (such as modulus G) can be affected by several factors, such as crystallinities [33, 34], temperature [35] and most importantly entanglement density [36]. The model also 2
shows mathematical simplicity that a plot of σ against λ −λ
−1
gives linear segment at strain range beyond yield
with the slop of the linear dependency being the modulus of the stretched Gaussian network. The application of this Haward-Thackray model to PCT implies some simplification and idealization. As a polyester, PCT possesses also high molecular rigidity, strong intermolecular interaction and less entanglement. It is clear that during tensile deformation, chain slippage between rigid segments may occur which will affect the macroscopic apparent network properties. Nevertheless, this model provided a simple and meaningful approach for us to access fundamental properties of stretched chain networks. All possible complications can be summarized into the 2
apparent network properties. With this in mind, we examine in Figure 2 top, selected σ against λ −λ
−1
curves of
PCT samples stretched at different temperatures. Clearly, the model indeed grasps the main feature of the tensile stress-strain behavior of PCT in a wide temperature range. When being stretched at temperature below 80 oC, a straight line can be observed after yielding point with slop decreasing with the increasing of stretching temperature. This is caused by a gradual weakening of the intermolecular interaction with the increase of stretching temperature which then introduce a weakening of the network by slippage between chain segments. When the stretching temperature was 80 oC or higher, one observed an additional linear dependency at larger strain regime indicating that the sample exhibited tensile stress-strain behavior of two Gaussian networks of different moduli. The end of the stretching of the first Gaussian network and the beginning of the second one can be determined by the cross point of the two tangent lines representing the two networks (H-point). Thus determined H-point is similar but different to the strain-hardening point determined in the stress-strain curves. In our case, the determination of H-points was based on rubber stretching curves (plots of σ against λ2−λ−1). Our approach differentiates network properties. The occurrence of H-point indicates a change of network modulus during stretching whereas in stress-strain curves (σ vs ε)one would observe strain-hardening point even without a change in network modulus. A complete set of rubber stretching plots at different temperatures and an enlarged curve showing stretching behavior at low temperatures were given in Figure S2 in the supporting information documents. The strain values of H-point were collected as a function of stretching temperature in the middle of Figure 2. The strain at H-point increased with the increase of stretching temperature indicating that the PCT polymer softened with the increase of stretching temperature. This is attributed to an increased chain relaxation with temperature increase, which is detrimental for the chain orientation and the onset of H-point. The two moduli before and after the H-point showed obvious stretching temperature dependencies being decrease with the increase of temperature for the modulus of the first Gaussian network before H-point and a first increase then a decrease for the modulus of the network after H-point. Our results indicate that the amorphous PCT chains and temperature domains together have determinable effect on the deformation mechanism in its plastic deformation process. Clearly, strain-induced crystallization could occur when the samples were deformed around and above glass transition temperature so that the mechanical response can be largely altered. It is thus necessary to investigate possible crystallization behavior during stretching.
Figure 2. Mechanical data in the format of network stretching: true stress vs λ2 − 1/λ curves (top); The hardening point (H-point) against tensile temperature (middle) obtained from the intersection of the two tangent lines in top figure; Modulus G against tensile temperature of PCT stretched at different temperature (bottom) calculated from the slop of two tangent lines before and after H-point.
Strain-Induced Crystallization by WAXS In order to understand the temperature dependent deformation mechanism of amorphous PCT, we performed in situ WAXS measurements during stretching at different temperatures. The true stress-strain curves as well as selected WAXS patterns taken during uniaxial tensile stretching of PCT at indicated temperatures are given in Figure 3 (More WAXS patterns obtained at other temperatures during deformation process in Figure S3 of supporting information). The true stress-strain curves obtained under different temperatures exhibited distinct behavior of strain hardening and strain at break. Clearly, for samples deformed at different temperatures one observed different WAXS patterns even at the same strain. This indicates that the deformation induced structural change depended heavily on stretching temperature. As the time needed for obtaining one WAXS image was 23 s,
an average structural changed in less than 5% strain span was recorded in each WAXS pattern. In Figure 3, average values of strain were given under the WAXS patterns. Obviously, the WAXS images changed from an initially isotropic pattern before deformation to anisotropic ones after stretching. An example of using halo method to separate the WAXS patterns was given in Figure S4 of the supporting information. Figure 4 showed the 1D WAXS curves from the isotropic and anisotropic patterns. Data in Figure 4 clearly indicate that the unoriented crystals appeared at stretching temperature above 80 oC and oriented crystals appeared at temperature above 90 oC. According to the method described in Figures S4 and S5 in supporting information, the fractions of the oriented crystals, unoriented crystals and anisotropic fraction (containing oriented amorphous and oriented crystalline phase) were estimated and shown in Figure 5. The strain dependent mass fraction of anisotropic phase present in Figure 5 (top), showed that the undeformed sample was a little oriented due to the compression-molding process. The fraction of oriented phase increased with the increase of strain at all temperatures indicating the development of chain orientation along the stretching direction. The results of above in-situ WAXS analysis allow us to divide the stretching temperature into three zones with boundaries at 80 and 90 oC according to structural development during uniaxial deformation. In zone Ι below 80 oC, there was only the orientation of molecular chain in amorphous phase during deformation process in PCT samples. Figure 3 (top) showed the results of structural evolution during tensile deformation process at 25 oC. It is clear that the WAXS pattern present only an amorphous halo scattering indicating the initial structure in the quenched PCT sample was amorphous and without any preferred orientation before stretching at 25 oC. In the process of stretching, an increase in stress with strain was observed without distinct necks and strain hardening. The intensity of the amorphous halo concentrates more and more on the meridian direction, that is, at right angles to the tensile direction with the strain increase, which is the preferential orientation of the molecular chain along stretching direction causing an azimuthal redistribution of the intensity in the scattering pattern. In zone ΙΙ between 80 and 90 oC, one observed firstly a gradual orientation of the amorphous phase followed by the generation of very small amounts of crystals during deformation process. The structural development during stretched process of the PCT samples measured at 90 oC was shown Figure 3 (middle). Similarly, the WAXS pattern was isotropic before stretching and the amorphous halo become focused on the meridian with the strain increase. However, we found that the diffraction peak of crystallographic (103) plane in the equator direction appeared at the strain value of 1.5 suggesting the presence of stain-induced crystallization of amorphous PCT during stretching at 90 oC. Scattering intensity profiles for both equatorial and meridional directions derived from the WAXS patterns of PCT stretched at 25 to 108 oC are shown in Figure S6 in supporting information document. Obviously, from the equator profiles one identifies distinct reflection peak during deformation process at 80 oC-90 oC, which can be indexed as the (103) reflection based on the triclinic unit cell structure in PCT[37]. However, no diffraction peak can be identified from the meridian profile. We may understand this phenomenon based on results obtained on poly(ethylene terephthalate) (PET). Hsiao et al.[26,38] studied the structural formation of amorphous PET and found that the appearance of the (103) peak in the equator profile was due to the perfection and orientation increase of the generated crystals during tensile stretching above glass temperature. They also proposed that a strong (010) peak appeared near the meridian was due to the inter-chain benzene sheet structure formation rather than a stacking structure of benzene. The sheet structure of benzene means that the benzene ring planes are on the same plane whereas the stacking structure of benzene means that the benzene ring planes pile up as layers. Based on their model and our experimental results, we could conclude that the initial crystalline structure generated during stretching process between 80 oC to 90 oC was dominated by the formation of a benzene stacking structure that results in no distinct diffraction peak in the meridian profile except a (103) peak in the equator profile. In zone III above 90 oC, upon a critical strain the PCT samples started to generate crystallites of triclinic unit cell during stretching process. Figure 3 (bottom) showed the true stress-strain curve and select 2D WAXS patterns of
amorphous PCT stretched at 108 oC. The WAXS patterns changed from isotropic amorphous halo scattering to two concentrated arcs at the meridian at small strain regime suggesting the preferential orientation of polymer chains along stretching direction. With the increasing of strain, the scattering arcs in the meridian developed gradually becoming stronger and narrower and then into three peaks of (010), (110) and (100) planes. The 1D WAXS curves in equatorial and meridional direction in Figure S6 (supporting information) at temperature above 92 oC, exhibited two strong peaks of (010) and (110) reflections near the equator. Such results indicate that the initial crystalline structure was due to the formation of inter-chain sheet structure of benzene. The different temperature stretched samples were further examined by DSC heating scans to compare their thermal properties. Such results were collected in Figure 6 together with a heating scan curve of original undeformed amorphous sample. The amorphous sample showed typical cold crystallization occurring at 120 to 130 oC during heating and a final melting at around 290 oC. The areas under the cold crystallization and melting peaks were very close with each other indicating a nearly complete amorphous nature of the sample at low temperatures. Interestingly, the DSC heating curves of samples stretched at 25 and 80 oC showed notable results. For the 25 oC stretched sample, no strain-induced crystallization was observed in WAXS experiments but the area of the melting peak was much larger than the cold crystallization peak in the DSC curve. For 80 oC stretched sample, only very small fraction of crystals was observed in WAXS experiments but the DSC heating curves showed an absence of cold crystallization peak and a seemingly even larger melting peak compared to other samples. The result indicates that strain energy generated during stretching was effectively stored in the system that converted into crystallization without giving obvious thermal effect during heating. Similar results have been also observed in stretched PET[39,40]. For the sample stretched at 100 oC, it showed a common melting behavior of strain-induced crystallized sample. The results also indicate that the PCT sample under investigation had a glass transition temperature of around 80 to 100 oC.
Figure 3. Selected WAXS images during the collection of true stress-strain curve at indicated temperatures. Each image was taken at the average strain indicated by the value on the graph.
Figure 4. 1D WAXS curves taken from the separated isotropic (left) and anisotropic (right) patterns at indicated temperature.
Figure 5. Changes of anisotropic fraction (top), oriented crystals (middle) and unoriented crystals (bottom) as a function of strain for samples stretched at different temperatures. Data obtained via fitting corresponding 1D WAXS curves.
Figure 6. DSC heating curves of undeformed and different temperature stretched PCT.
Correlation Between Crystal Orientation And Mechanical Properties. As reported in the literature, the stress-strain curve can be divided into three regimes being regime I before strain-hardening, regime II strain-hardening, and regime III after strain hardening. Regime I was attributed to the orientation of chains by Billon et al. [41]. Crystal nucleation was then occurred in regime II which was followed by crystal growth in regime III. However, formation of mesomorphic phase in regime I was proposed by Hsiao et al.[25,26,38]. They attributed strain-hardening to the development of network built up by imperfect crystals and mesomorphic phase. They assumed a continuous crystal growth and stabilization in regime III. There is one character in common for both models mentioned above namely the occurrence of chain orientation before crystallization which is also consistent with our observations. With the aids of above detailed analysis of in situ WAXS data and mechanical results, we are now able to discuss the micro-structural origin of the changes in mechanical properties during stretching of the amorphous PCT samples at different temperatures. Figure 7 plotted again the strain at the H-point (the onset of the second Gaussian network stretching on the stress-strain curve determined by the intersection of two tangential lines along the two straight segments in rubber stretching curves (Figure 2 (top)) together with the onset of unoriented crystals and oriented crystals. Clearly, the onset of unoriented and oriented crystals occurred in general at different strains and showed different stretching temperature dependencies. The strain of onset of unoriented crystals increased with the stretching temperature whereas that of oriented crystals first decreased with the stretching temperature and then increased when the stretching temperature was higher than102 oC. In addition, unoriented crystals started to appear already at a stretching temperature of 80 oC whereas below 85 oC, no oriented crystals were observed. The most surprising result in Figure 7 is that the onset strain of unoriented crystals perfectly overlapped with that of H-point. This implies
strongly that the change of the mechanical properties of the samples from a rather soft network with smaller modulus to a much stronger ones with higher modulus is due to the occurrence of unoriented crystals during stretching. In the low stretching temperature regime below 90 oC, strain induced crystallization first produced unoriented crystals which induced an obvious hardening of the system (initiating of the second stronger network). Oriented crystals were not developed due to early breakage of the samples at stretching temperature near glass transition (below 85 oC). With the increase of stretching temperature, oriented crystals could be generated upon further stretching the already crystallized system. According to results present in Figure 5, the fraction of both unoriented crystals and oriented ones increases with the increase in strain. It is therefore not straightforward to assign the oriented crystals to direct strain-induced crystallization of the oriented amorphous chain segments or reorientation of the already existing unoriented crystals or both. When the stretching temperature was higher than 90 oC, oriented crystals, although showed complicated stretching temperature dependency of first a decrease then an increase in onset strain, they appeared always before the occurrence of unoriented crystals. Clearly, the oriented crystals have been oriented with the chain direction along the stretching direction and cannot be re-distributed into an isotropic orientation under the condition of continuous uniaxial stretching. The first decrease of onset strain of the oriented crystals as a function of stretching temperature must be related to the gradual de-frozen of the system around glass transition temperature. Increased segmental mobility at higher stretching temperature facilitated the nucleation of oriented crystals. After glass transition, chain segments became much mobile so that a higher strain was needed to fix them for nucleation of oriented crystals. The observed unoriented crystals, which are decisive for changing the modulus of the network, must be generated out of the amorphous network. The insensitive nature of the network modulus on oriented crystals provides indispensable information on understanding the physical origin of the mechanical properties in similar polymeric systems under tensile stretching. The result first of all indicates that the mechanical response of the amorphous network was not affected by the generation of crystals with chain segments aligned along the stretching direction, i.e. they did not change the number of effective junction points that determined the network modulus. The unoriented crystals, on the other hand, altered the network structure very much by increasing of the number of effective junction points so that an increase in network modulus.
Figure 7. Stretching temperature dependent critical strains for the appearance of oriented and unoriented crystals and H-points.
It must be mentioned that the absolute crystallinity as well as the fractions of oriented and unoriented crystals at the H-point might be temperature dependent. With the aid of data present in Figure 5 as well as Figure S7 in the supporting information document we collected such data in Figure 8. One observes first of all a constant value below Tg and a general increase in crystallinity at H-point when the sample was deformed above Tg. As was discussed before, no oriented crystals were observed at H-point below Tg (they appear later). The fraction of unoriented crystals at H-point remained at a certain low level through out the whole temperature range investigated. This result again suggests that the occurrence of oriented crystals with chain segments along stretching direction during stretching does not change the modulus of the stretched network. They do show clear effect on the stress-strain curves giving obvious strain-hardening phenomenon. However, such strain-hardening behavior does not necessarily indicate a change in network modulus. What we discussed in this work is about the change in network property (namely modulus of the network) which can only be identified using plots of rubber stretching (stress
vs rubber extension
).
Figure 8. Stretching temperature dependent crystallinity, fractions of unoriented crystals and oriented crystals at H-point.
Conclusions In summary, we have investigated strain-induced crystallization in amorphous PCT by means of in situ WAXS techniques. A model considering Gaussian networks was applied to treat the stress-strain behavior during stretching of the PCT samples at different temperatures. The WAXS data were analyzed in such a way that mass fractions of anisotropic phase, oriented crystals and unoriented crystals were decomposed from the total 2D scattering patterns. Macroscopically, the samples showed mechanical behavior of two Gaussian networks when stretched at temperatures above 80 oC where strain-induced crystallization started to appear. The switch of the two Gaussian networks can be easily observed from a plot of true stress against rubber extension 2
(λ −λ
−1
two straight segments can be clearly identified. The intersection point of the tangential line
along the two straight segments denoting the macroscopic hardening point at a particular stretching temperature coincided with the onset of crystallization of unoriented crystals irrespective of the occurrence or not and the onset of the oriented crystals.
ACKNOWLEDGEMENTS This work is financially supported by the National Natural Science Foundation of China (51525305). We acknowledge Prof. Zhonghua Wu and Dr. Guang Mo for their help during synchrotron X-ray scattering
measurements.
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Highlights In-situ WAXS measurements were performed during stretching of PCT at different temperatures. Gaussian network stretching of Haward-Thackray model describes the mechanical data very well. Change in network modulus was observed when unoriented crystals were generated during stretching.
Author statement Zhongshuo Miao: Validation, Investigation, Data Curation, Writing-Original Draft Yongfeng Men: Conceptualization, Resources, Writing-Review & Editing, Supervision, Project administration, Funding acquisition
Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0032-3861(19)31044-4
DOI:
https://doi.org/10.1016/j.polymer.2019.122038
Reference:
JPOL 122038
To appear in:
Polymer
Received Date: 4 October 2019 Revised Date:
22 November 2019
Accepted Date: 28 November 2019
Please cite this article as: Miao Z, Men Y, Temperature dependent network properties of amorphous PCT during tensile stretching, Polymer (2019), doi: https://doi.org/10.1016/j.polymer.2019.122038. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Graphic Abstract
Temperature Dependent Network Properties of Amorphous PCT during Tensile Stretching Zhongshuo Miao, Yongfeng Men* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, University of Chinese Academy of Sciences, Renmin Street 5625, 130022 Changchun, P.R. China
ABSTRACT: Tensile deformation behavior of amorphous Poly (1,4-cyclohexylenedimethylene Terephthalate) (PCT) at 25 oC to 108 oC was investigated by means of in situ synchrotron wide-angle X-ray scattering (WAXS). WAXS patterns were processed in a way that the scatterings from isotropic and anisotropic structures were separated. It turned out that the influences of temperature during deformation can be divided into three zones: below 80 oC, only molecular chains oriented; between 80 oC to 90 oC, very few imperfect crystals were induced by strain; above glass transition temperature (92 oC), the triclinic PCT crystalline structure began to form during deformation process. It was found that the Gaussian model of Haward and Thackray described the stress-strain behavior of the tensile stretched amorphous PCT very well. However, when being stretched above 80 oC, the system showed two characteristic network moduli because of strain-induced crystallization. The onset of strain-hardening occurred immediately after the formation of unoriented crystalllites which served as physical cross-linking points to reinforce the material.
Key Words: network; tensile stretching; semi-crystalline polymers; X-ray diffraction; stress-strain curve
INTRODUCTION Compared to other thermoplastics, such as polyethylene terephthalate and Polybutylene terephthalate, Poly(1,4-cyclohexanedimethylene terephthalate) (PCT) has excellent thermal, hydrolysis stability and electrical properties[1-3]. The application of PCT has been expanded to films and fibers due to its extremely low oligomer content and good chemical resistance[3,4]. Despite its high application value, only few researches concerned the physical properties of PCT, like crystallization and mechanical properties[5,6]. Considering the crystallization behavior,
random
copolymers
terephthalate)(P(ET-co-CT)), (P(BT-co-CT)),
such
as
poly(ethylene
terephthalate-co-1,4-cyclohexylenedimethylene
poly(butyleneterephthalate-co-1,4-
poly(1,4-cyclohexylene
dimethylene
cyclohexanedimethylene
terephthalate)
terephthalate-co-hex-amethylene
terephthalate)
(P(CT-co-HT)), poly(trimethylene terephthalate-co-1,4-cyclohexylene dimethylene terephthalate) (PTCT) were found to exhibit composition dependent eutectic melting and isodimorphic crystallization behavior[7-13]. It was also discovered that PCT showed mechanically more ductile property than PET, which has its origin of the existence of cooperative secondary relaxation process because of the conformation transition of cyclohexylene rings from boat to chair. In many cases, PCT may end up with a quasi-amorphous structure after production due to its high glass transition temperature (Tg) being much higher than room temperature. Further mechanical treatment may lead to a continuous crystallization so that change in final properties. Therefore, strain-induced crystallization is of particular importance in the research of physical properties of PCT. The first strain-induced crystallization phenomenon in polymers may be dated back to 1925 when Katz investigated natural rubber (NR)[14]. Mechanical properties of NR can be strongly affected by the strain-induced crystallization, especially the crack propagation resistance[15,16.17] and fatigue performance[18]. Based on the development of thermal measurements and in situ wide-angle X-ray scattering (WAXS) technologies, many
literatures have reported the explanation of the mechanism of strain-induced crystallization, and provided important knowledge on crystallinity and size and orientation of the crystallites[14,19,20]. Flory[21] and Toki[22] considered that the crystallites induced by the strain were extended chain crystals which led to a reduction of the stress. Consequent crystal growth was concluded to be parallel to the drawing direction. The finite extensibility of the polymer chain network was thought to be the reason of the increase in crystallinity and stress. Tosaka et al.[23] reported network chain density independency of the onset strain of deformation induced crystallization. Clearly, stretched molecular chains seemed to act as nuclei while the crystal growth must be contributed by the involvement of the surrounding chains. Deformation caused structural variations in amorphous PET below and above Tg have been investigated extensively. Blundell et al.[24] observed the appearance of a structure of smectic A during fast tensile deformation of PET. The smectic A structure was proposed to be a precursor for further crystallization because the triclinic crystalline diffraction peak appeared simultaneously with the disappearance of smectic A one. Moreover, a smectic C structure was reported to appear during tensile stretching of amorphous PET at 50 °C below Tg by Ran et al.[25]. The mesophase was found to develop immediately after necking. The mesophase presented a sharp meridional peak 001′with a d spacing of 0.032 nm. This d spacing is smaller than the length of a monomer in typical triclinic unit cell. It was then assumed that the chain segments in mesophase gave a tilted smectic C structure. Stress-strain behavior of amorphous PET above Tg has been also connected to the structural variation. It was shown that necking induced the formation of the mesophase immediately. Afterwards, a three-dimensional network composed of mesomorphic chains and imperfect crystals was formed in the strain-hardening regime followed by the formation of mesophase and stable crystals after strain-hardening[26].
Up to now, knowledge about
strain-induced structural evolution in amorphous PCT remains incomplete. In this work, we studied the structural evolution of amorphous PCT during tensile deformation at different temperatures by in situ synchrotron wide-angle X-ray scattering (WAXS) techniques. Mechanical response of the PCT samples at different temperatures has been successfully linked to the formation of crystallites during stretching.
EXPERIMENTAL SECTION Materials and Samples. The PCT polyester was an industrial material，trade name SkyPURA0302, produced by SK chemicals. It has a number-average (Mn) and weight-average molecule weight (Mw) of 22.5 and 48.4 kg/mol, respectively. A film with a 0.5 mm thickness was obtained by compression-molding the material at 300 °C. Thermal history of the material was removed by keeping the film at 300 °C for 5 min. The thus pressed film was then quenched into ice-water to obtain an amorphous PCT film. The “dog bone” tensile specimens with sizes of 25 × 8 × 0.5 mm3 were obtained using a punch. The samples were tensile stretched with a TST 350 tensile testing machine (Linkam, U.K.). To obtain exact strain values at the X-ray beam position, we took optical photo images of the sample during stretching. Hencky strain εH was used as a quantity of the extension. εH is defined as follows: εH = 2 ln(b0/b) where b0 and b are the widths of undeformed and deformed area, respectively.
Characterization. Synchrotron WAXS measurements were conducted at the beamline 1W2A, BSRF, Beijing. The beamline was running under an X-ray wavelength of 0.154 nm. To record WAXS patterns, we put a MAR CCD detector 83.6 mm away from the sample position. The detector has a resolution of 2048 × 2048 under pixel size of 79 µm. A constant crosshead speed of 10 µm/s at deformation temperatures of 25 to 110 °C was utilized. The primary X-ray beam was illuminated in the middle of the sample which was horizontally placed. The WAXS patterns were recorded during deformation process, and exposure time was 23s for every pattern.
In order to study the orientational behavior of crystals and amorphous phase, we implemented the “halo method” suggested by Hsiao et al. [27,28] for the analysis of 2D WAXS patterns. The distortion effect caused by flat-plate detector on the WAXS patterns was corrected using Fraser method[29]. In the “halo method”, azimuthal scans at different q values were collected to yield scattering intensity distributions. In each azimuthal scattering intensity distribution curve, the minimum intensity value was assigned to the isotropic scattering contribution at this particular q position. By subtracting thus obtained isotropic scattering intensity distribution from the complete WAXS pattern, we obtained anisotropic scattering pattern. In both isotropic part and anisotropic part, there are contributions from crystalline and amorphous phases. The mass fractions of crystals and amorphous phase both oriented and unoriented were quantitative evaluated using a peak fitting method[30]. Differential scanning calorimeter (DSC) measurements were carried out with a DSC1 Stare System (Mettler Toledo Swiss). The set-up was calibrated by using indium as a standard for temperature and melting enthalpy. A heating rate of 10 K/min was used during the measurements for PCT samples.
RESULTS AND DISCUSSIONS 1. Deformation Behaviors and Gaussian Model Selected true stress−strain curves of amorphous PCT polyester at different temperatures from 25 oC to 108 oC are summarized in Figure 1. A complete set of such curves at more deformation temperatures can be found in Figure S1 in supporting information document. First of all, the elastic deformation occurred only within a very low strain range of less than 0.05 at all temperatures. At stretching temperature below glass transition (90 oC), a strain softening with a slightly decrease of true stress took place after the yield point whereas no distinct yielding was observed above 90 oC. Before strain-hardening, the true stress dramatic decreases with the increase of temperature from 25 to 90 oC.
After strain-hardening, a rather similar slop was observed in the stress-strain curves at each
temperature. These behaviors reveal that the deformation properties of amorphous PCT are strongly influenced by the stretching temperature. Because the molecular chain is frozen below Tg, the movement of the molecular chain segments is limited when being stretched at temperatures below Tg. One would not expect too much ordering of the chain segments into crystalline structure. Strain-induced crystallization, however, can take place when the material is stretched at temperatures above Tg. This behavior is reflected also onto the macroscopic stress-strain dependency. The stress-strain curves for samples stretched below Tg show a rather strong increase in stress before yielding whereas they exhibit much larger strain softening behavior after yielding. Eventually, the stress increases significantly at large strains indicating the occurrence of strain-induced crystallization when the samples were stretched above Tg.
Figure 1. True stress−strain curves of stretched PCT samples at different temperature.
In order to investigate network properties of PCT samples during stretching at different temperatures, a model developed by Haward and Thackray [31,32] can be applied. The model was previously developed to describe tensile stress-strain behavior of glassy polymers and has also been successfully applied to semi-crystalline polymers. The model is built upon three elements with an Eyring dashpot being in parallel with a Gaussian rubbery spring which are then in series with a Hookean spring. The model implies that at large strain regimes the deformation of semicrystalline polymers should be determined by the stretching of the Gaussian rubbery spring network. The property of this Gaussian spring network (such as modulus G) can be affected by several factors, such as crystallinities [33, 34], temperature [35] and most importantly entanglement density [36]. The model also 2
shows mathematical simplicity that a plot of σ against λ −λ
−1
gives linear segment at strain range beyond yield
with the slop of the linear dependency being the modulus of the stretched Gaussian network. The application of this Haward-Thackray model to PCT implies some simplification and idealization. As a polyester, PCT possesses also high molecular rigidity, strong intermolecular interaction and less entanglement. It is clear that during tensile deformation, chain slippage between rigid segments may occur which will affect the macroscopic apparent network properties. Nevertheless, this model provided a simple and meaningful approach for us to access fundamental properties of stretched chain networks. All possible complications can be summarized into the 2
apparent network properties. With this in mind, we examine in Figure 2 top, selected σ against λ −λ
−1
curves of
PCT samples stretched at different temperatures. Clearly, the model indeed grasps the main feature of the tensile stress-strain behavior of PCT in a wide temperature range. When being stretched at temperature below 80 oC, a straight line can be observed after yielding point with slop decreasing with the increasing of stretching temperature. This is caused by a gradual weakening of the intermolecular interaction with the increase of stretching temperature which then introduce a weakening of the network by slippage between chain segments. When the stretching temperature was 80 oC or higher, one observed an additional linear dependency at larger strain regime indicating that the sample exhibited tensile stress-strain behavior of two Gaussian networks of different moduli. The end of the stretching of the first Gaussian network and the beginning of the second one can be determined by the cross point of the two tangent lines representing the two networks (H-point). Thus determined H-point is similar but different to the strain-hardening point determined in the stress-strain curves. In our case, the determination of H-points was based on rubber stretching curves (plots of σ against λ2−λ−1). Our approach differentiates network properties. The occurrence of H-point indicates a change of network modulus during stretching whereas in stress-strain curves (σ vs ε)one would observe strain-hardening point even without a change in network modulus. A complete set of rubber stretching plots at different temperatures and an enlarged curve showing stretching behavior at low temperatures were given in Figure S2 in the supporting information documents. The strain values of H-point were collected as a function of stretching temperature in the middle of Figure 2. The strain at H-point increased with the increase of stretching temperature indicating that the PCT polymer softened with the increase of stretching temperature. This is attributed to an increased chain relaxation with temperature increase, which is detrimental for the chain orientation and the onset of H-point. The two moduli before and after the H-point showed obvious stretching temperature dependencies being decrease with the increase of temperature for the modulus of the first Gaussian network before H-point and a first increase then a decrease for the modulus of the network after H-point. Our results indicate that the amorphous PCT chains and temperature domains together have determinable effect on the deformation mechanism in its plastic deformation process. Clearly, strain-induced crystallization could occur when the samples were deformed around and above glass transition temperature so that the mechanical response can be largely altered. It is thus necessary to investigate possible crystallization behavior during stretching.
Figure 2. Mechanical data in the format of network stretching: true stress vs λ2 − 1/λ curves (top); The hardening point (H-point) against tensile temperature (middle) obtained from the intersection of the two tangent lines in top figure; Modulus G against tensile temperature of PCT stretched at different temperature (bottom) calculated from the slop of two tangent lines before and after H-point.
Strain-Induced Crystallization by WAXS In order to understand the temperature dependent deformation mechanism of amorphous PCT, we performed in situ WAXS measurements during stretching at different temperatures. The true stress-strain curves as well as selected WAXS patterns taken during uniaxial tensile stretching of PCT at indicated temperatures are given in Figure 3 (More WAXS patterns obtained at other temperatures during deformation process in Figure S3 of supporting information). The true stress-strain curves obtained under different temperatures exhibited distinct behavior of strain hardening and strain at break. Clearly, for samples deformed at different temperatures one observed different WAXS patterns even at the same strain. This indicates that the deformation induced structural change depended heavily on stretching temperature. As the time needed for obtaining one WAXS image was 23 s,
an average structural changed in less than 5% strain span was recorded in each WAXS pattern. In Figure 3, average values of strain were given under the WAXS patterns. Obviously, the WAXS images changed from an initially isotropic pattern before deformation to anisotropic ones after stretching. An example of using halo method to separate the WAXS patterns was given in Figure S4 of the supporting information. Figure 4 showed the 1D WAXS curves from the isotropic and anisotropic patterns. Data in Figure 4 clearly indicate that the unoriented crystals appeared at stretching temperature above 80 oC and oriented crystals appeared at temperature above 90 oC. According to the method described in Figures S4 and S5 in supporting information, the fractions of the oriented crystals, unoriented crystals and anisotropic fraction (containing oriented amorphous and oriented crystalline phase) were estimated and shown in Figure 5. The strain dependent mass fraction of anisotropic phase present in Figure 5 (top), showed that the undeformed sample was a little oriented due to the compression-molding process. The fraction of oriented phase increased with the increase of strain at all temperatures indicating the development of chain orientation along the stretching direction. The results of above in-situ WAXS analysis allow us to divide the stretching temperature into three zones with boundaries at 80 and 90 oC according to structural development during uniaxial deformation. In zone Ι below 80 oC, there was only the orientation of molecular chain in amorphous phase during deformation process in PCT samples. Figure 3 (top) showed the results of structural evolution during tensile deformation process at 25 oC. It is clear that the WAXS pattern present only an amorphous halo scattering indicating the initial structure in the quenched PCT sample was amorphous and without any preferred orientation before stretching at 25 oC. In the process of stretching, an increase in stress with strain was observed without distinct necks and strain hardening. The intensity of the amorphous halo concentrates more and more on the meridian direction, that is, at right angles to the tensile direction with the strain increase, which is the preferential orientation of the molecular chain along stretching direction causing an azimuthal redistribution of the intensity in the scattering pattern. In zone ΙΙ between 80 and 90 oC, one observed firstly a gradual orientation of the amorphous phase followed by the generation of very small amounts of crystals during deformation process. The structural development during stretched process of the PCT samples measured at 90 oC was shown Figure 3 (middle). Similarly, the WAXS pattern was isotropic before stretching and the amorphous halo become focused on the meridian with the strain increase. However, we found that the diffraction peak of crystallographic (103) plane in the equator direction appeared at the strain value of 1.5 suggesting the presence of stain-induced crystallization of amorphous PCT during stretching at 90 oC. Scattering intensity profiles for both equatorial and meridional directions derived from the WAXS patterns of PCT stretched at 25 to 108 oC are shown in Figure S6 in supporting information document. Obviously, from the equator profiles one identifies distinct reflection peak during deformation process at 80 oC-90 oC, which can be indexed as the (103) reflection based on the triclinic unit cell structure in PCT[37]. However, no diffraction peak can be identified from the meridian profile. We may understand this phenomenon based on results obtained on poly(ethylene terephthalate) (PET). Hsiao et al.[26,38] studied the structural formation of amorphous PET and found that the appearance of the (103) peak in the equator profile was due to the perfection and orientation increase of the generated crystals during tensile stretching above glass temperature. They also proposed that a strong (010) peak appeared near the meridian was due to the inter-chain benzene sheet structure formation rather than a stacking structure of benzene. The sheet structure of benzene means that the benzene ring planes are on the same plane whereas the stacking structure of benzene means that the benzene ring planes pile up as layers. Based on their model and our experimental results, we could conclude that the initial crystalline structure generated during stretching process between 80 oC to 90 oC was dominated by the formation of a benzene stacking structure that results in no distinct diffraction peak in the meridian profile except a (103) peak in the equator profile. In zone III above 90 oC, upon a critical strain the PCT samples started to generate crystallites of triclinic unit cell during stretching process. Figure 3 (bottom) showed the true stress-strain curve and select 2D WAXS patterns of
amorphous PCT stretched at 108 oC. The WAXS patterns changed from isotropic amorphous halo scattering to two concentrated arcs at the meridian at small strain regime suggesting the preferential orientation of polymer chains along stretching direction. With the increasing of strain, the scattering arcs in the meridian developed gradually becoming stronger and narrower and then into three peaks of (010), (110) and (100) planes. The 1D WAXS curves in equatorial and meridional direction in Figure S6 (supporting information) at temperature above 92 oC, exhibited two strong peaks of (010) and (110) reflections near the equator. Such results indicate that the initial crystalline structure was due to the formation of inter-chain sheet structure of benzene. The different temperature stretched samples were further examined by DSC heating scans to compare their thermal properties. Such results were collected in Figure 6 together with a heating scan curve of original undeformed amorphous sample. The amorphous sample showed typical cold crystallization occurring at 120 to 130 oC during heating and a final melting at around 290 oC. The areas under the cold crystallization and melting peaks were very close with each other indicating a nearly complete amorphous nature of the sample at low temperatures. Interestingly, the DSC heating curves of samples stretched at 25 and 80 oC showed notable results. For the 25 oC stretched sample, no strain-induced crystallization was observed in WAXS experiments but the area of the melting peak was much larger than the cold crystallization peak in the DSC curve. For 80 oC stretched sample, only very small fraction of crystals was observed in WAXS experiments but the DSC heating curves showed an absence of cold crystallization peak and a seemingly even larger melting peak compared to other samples. The result indicates that strain energy generated during stretching was effectively stored in the system that converted into crystallization without giving obvious thermal effect during heating. Similar results have been also observed in stretched PET[39,40]. For the sample stretched at 100 oC, it showed a common melting behavior of strain-induced crystallized sample. The results also indicate that the PCT sample under investigation had a glass transition temperature of around 80 to 100 oC.
Figure 3. Selected WAXS images during the collection of true stress-strain curve at indicated temperatures. Each image was taken at the average strain indicated by the value on the graph.
Figure 4. 1D WAXS curves taken from the separated isotropic (left) and anisotropic (right) patterns at indicated temperature.
Figure 5. Changes of anisotropic fraction (top), oriented crystals (middle) and unoriented crystals (bottom) as a function of strain for samples stretched at different temperatures. Data obtained via fitting corresponding 1D WAXS curves.
Figure 6. DSC heating curves of undeformed and different temperature stretched PCT.
Correlation Between Crystal Orientation And Mechanical Properties. As reported in the literature, the stress-strain curve can be divided into three regimes being regime I before strain-hardening, regime II strain-hardening, and regime III after strain hardening. Regime I was attributed to the orientation of chains by Billon et al. [41]. Crystal nucleation was then occurred in regime II which was followed by crystal growth in regime III. However, formation of mesomorphic phase in regime I was proposed by Hsiao et al.[25,26,38]. They attributed strain-hardening to the development of network built up by imperfect crystals and mesomorphic phase. They assumed a continuous crystal growth and stabilization in regime III. There is one character in common for both models mentioned above namely the occurrence of chain orientation before crystallization which is also consistent with our observations. With the aids of above detailed analysis of in situ WAXS data and mechanical results, we are now able to discuss the micro-structural origin of the changes in mechanical properties during stretching of the amorphous PCT samples at different temperatures. Figure 7 plotted again the strain at the H-point (the onset of the second Gaussian network stretching on the stress-strain curve determined by the intersection of two tangential lines along the two straight segments in rubber stretching curves (Figure 2 (top)) together with the onset of unoriented crystals and oriented crystals. Clearly, the onset of unoriented and oriented crystals occurred in general at different strains and showed different stretching temperature dependencies. The strain of onset of unoriented crystals increased with the stretching temperature whereas that of oriented crystals first decreased with the stretching temperature and then increased when the stretching temperature was higher than102 oC. In addition, unoriented crystals started to appear already at a stretching temperature of 80 oC whereas below 85 oC, no oriented crystals were observed. The most surprising result in Figure 7 is that the onset strain of unoriented crystals perfectly overlapped with that of H-point. This implies
strongly that the change of the mechanical properties of the samples from a rather soft network with smaller modulus to a much stronger ones with higher modulus is due to the occurrence of unoriented crystals during stretching. In the low stretching temperature regime below 90 oC, strain induced crystallization first produced unoriented crystals which induced an obvious hardening of the system (initiating of the second stronger network). Oriented crystals were not developed due to early breakage of the samples at stretching temperature near glass transition (below 85 oC). With the increase of stretching temperature, oriented crystals could be generated upon further stretching the already crystallized system. According to results present in Figure 5, the fraction of both unoriented crystals and oriented ones increases with the increase in strain. It is therefore not straightforward to assign the oriented crystals to direct strain-induced crystallization of the oriented amorphous chain segments or reorientation of the already existing unoriented crystals or both. When the stretching temperature was higher than 90 oC, oriented crystals, although showed complicated stretching temperature dependency of first a decrease then an increase in onset strain, they appeared always before the occurrence of unoriented crystals. Clearly, the oriented crystals have been oriented with the chain direction along the stretching direction and cannot be re-distributed into an isotropic orientation under the condition of continuous uniaxial stretching. The first decrease of onset strain of the oriented crystals as a function of stretching temperature must be related to the gradual de-frozen of the system around glass transition temperature. Increased segmental mobility at higher stretching temperature facilitated the nucleation of oriented crystals. After glass transition, chain segments became much mobile so that a higher strain was needed to fix them for nucleation of oriented crystals. The observed unoriented crystals, which are decisive for changing the modulus of the network, must be generated out of the amorphous network. The insensitive nature of the network modulus on oriented crystals provides indispensable information on understanding the physical origin of the mechanical properties in similar polymeric systems under tensile stretching. The result first of all indicates that the mechanical response of the amorphous network was not affected by the generation of crystals with chain segments aligned along the stretching direction, i.e. they did not change the number of effective junction points that determined the network modulus. The unoriented crystals, on the other hand, altered the network structure very much by increasing of the number of effective junction points so that an increase in network modulus.
Figure 7. Stretching temperature dependent critical strains for the appearance of oriented and unoriented crystals and H-points.
It must be mentioned that the absolute crystallinity as well as the fractions of oriented and unoriented crystals at the H-point might be temperature dependent. With the aid of data present in Figure 5 as well as Figure S7 in the supporting information document we collected such data in Figure 8. One observes first of all a constant value below Tg and a general increase in crystallinity at H-point when the sample was deformed above Tg. As was discussed before, no oriented crystals were observed at H-point below Tg (they appear later). The fraction of unoriented crystals at H-point remained at a certain low level through out the whole temperature range investigated. This result again suggests that the occurrence of oriented crystals with chain segments along stretching direction during stretching does not change the modulus of the stretched network. They do show clear effect on the stress-strain curves giving obvious strain-hardening phenomenon. However, such strain-hardening behavior does not necessarily indicate a change in network modulus. What we discussed in this work is about the change in network property (namely modulus of the network) which can only be identified using plots of rubber stretching (stress
vs rubber extension
).
Figure 8. Stretching temperature dependent crystallinity, fractions of unoriented crystals and oriented crystals at H-point.
Conclusions In summary, we have investigated strain-induced crystallization in amorphous PCT by means of in situ WAXS techniques. A model considering Gaussian networks was applied to treat the stress-strain behavior during stretching of the PCT samples at different temperatures. The WAXS data were analyzed in such a way that mass fractions of anisotropic phase, oriented crystals and unoriented crystals were decomposed from the total 2D scattering patterns. Macroscopically, the samples showed mechanical behavior of two Gaussian networks when stretched at temperatures above 80 oC where strain-induced crystallization started to appear. The switch of the two Gaussian networks can be easily observed from a plot of true stress against rubber extension 2
(λ −λ
−1
two straight segments can be clearly identified. The intersection point of the tangential line
along the two straight segments denoting the macroscopic hardening point at a particular stretching temperature coincided with the onset of crystallization of unoriented crystals irrespective of the occurrence or not and the onset of the oriented crystals.
ACKNOWLEDGEMENTS This work is financially supported by the National Natural Science Foundation of China (51525305). We acknowledge Prof. Zhonghua Wu and Dr. Guang Mo for their help during synchrotron X-ray scattering
measurements.
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Highlights In-situ WAXS measurements were performed during stretching of PCT at different temperatures. Gaussian network stretching of Haward-Thackray model describes the mechanical data very well. Change in network modulus was observed when unoriented crystals were generated during stretching.
Author statement Zhongshuo Miao: Validation, Investigation, Data Curation, Writing-Original Draft Yongfeng Men: Conceptualization, Resources, Writing-Review & Editing, Supervision, Project administration, Funding acquisition
Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: