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Effects of chain conformation on the viscoelastic properties of polyacrylonitrile gels under large amplitude oscillatory shear
European Polymer Journal 85 (2016) 341–353
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Effects of chain conformation on the viscoelastic properties of polyacrylonitrile gels under large amplitude oscillatory shear Youngho Eom, Byoung Chul Kim ⇑ Department of Organic and Nano Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea
a r t i c l e
i n f o
Article history: Received 18 September 2016 Received in revised form 17 October 2016 Accepted 21 October 2016 Available online 26 October 2016 Keywords: Polyacrylonitrile gel Conformational change Aging Viscoelasticity Large amplitude oscillatory shear
a b s t r a c t The viscoelastic properties of polyacrylonitrile (PAN) gels prepared from the 20 wt% solutions in N,N-dimethyl formamide (DMF) were investigated under large amplitude oscillatory shear (LAOS) in terms of aging temperature of the solutions from 30 to 100 °C. Aging of PAN solutions reduced intrinsic viscosity and hydrodynamic diameter with little degradation. In the UV–vis spectra, the absorption peak at 268 nm represented the dipoledipole pairs of physically associated nitrile groups, whose intensity increased with aging temperature. These results confirmed that aging of the solutions gave rise to the contracted chain conformation with anti-parallel orientation-dominant nitrile groups. In 20 wt%, aged solutions showed gelation behavior more rapid than fresh one during storage at 25 °C. At the imposed strain amplitude of 400% in the LAOS measurement, PAN gel prepared from fresh solution gave sinusoidal waveform signal of stress and ellipse-shape Lissajous curve. However, gels from the solutions aged above 60 °C showed non-sinusoidal stress waveform and characteristic Lissajous pattern, representing strain-stiffening behavior. In addition, the gel of the solution aged at 100 °C gave the greater strain-stiffening ratio than the initial value after the successive LAOS tests at 80 and 30 °C, indicating the thermo-irreversibility. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Polyacrylonitrile (PAN) is one of the most studied polymers owing to the highly polar chemical structure as well as various applications such as apparel and industrial fibers [1–4]. As well recognized, the high polarity of nitrile groups makes the polymer infusible in the bulk state and resulting dipole-dipole interactions in the solutions bring about the complicated phase change and gelation with processing and environmental conditions [5–12]. In fact, the intermolecular interactions between nitrile groups are profoundly affected by their orientation modes depending on the chain conformation [13,14]. Hence, the chain conformation and corresponding orientation of nitrile groups have been explored both theoretically and empirically by using the computational simulation of the model structure, NMR, FT-IR, and X-ray techniques in the solid state, and light scattering and viscosity measurements in the solution state, respectively [15–23]. In theory, PAN chain produces the planar-zigzag or helix structure according to the nitrile group orientation but gives the irregular helix structure in practice [21]. The correlations between the chain conformation and structural characters of PAN fibers have been extensively investigated to obtain the desirable mechanical properties [24–27]. In comparison with the bulk PAN and its fibers, little literature on the effects of the chain conformation on the physical properties of the solutions and gels is available because ⇑ Corresponding author. E-mail address: [email protected] (B.C. Kim). http://dx.doi.org/10.1016/j.eurpolymj.2016.10.037 0014-3057/Ó 2016 Elsevier Ltd. All rights reserved.
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dipole-dipole interactions between polymer and solvent give rise to very complicated polymer structure. However, understanding the dependence of the solution and gel properties on the polymer structure offers critical information on the analysis of the phase changes of the systems due to the direct correlation between the conformation and interaction mode of nitrile groups. The conformational change of PAN in N,N-dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) was explored with respect to the solubility and temperature change [28]. In this study, we found that PAN gave different chain conformations in the fresh and aged solutions and corresponding gels. Moreover, such structural difference had significant effects on the rheological properties particularly for the gels. That is, the chain conformation would be closely related to the entanglement behavior and gel structure. Hence, the viscoelastic properties of PAN gels prepared from the 20 wt% solutions with different aging temperature ranging from 30 to 100 °C were assessed using the large amplitude oscillatory shear flow. 2. Experimental 2.1. Materials PAN was purchased from Sigma-Aldrich (USA), whose weight average molecular weight was 150,000. EP grade of DMF was purchased from Duksan Co. (Korea). PAN and DMF were used without further purification. PAN was vacuum dried at 60 °C for 24 h and dissolved in the solvents with stirring at 60 °C for 3 h until the solution became optically transparent. Before solution aging, all PAN solutions were purged with nitrogen at 25 °C for 15 min to minimize the oxidative degradation. The nitrogen purged solutions were aged over the temperature and time ranges from 30 to 160 °C and 5 min to 24 h, respectively. 2.2. Measurement of physical properties The FT-IR spectra of PAN solutions in DMF were obtained by using 20-times reflectance ATR-FTIR (Ge crystal and incident angle of 45°) with heating controller. All temperature-dependent spectra were recorded with a spectral resolution of 4 cm1 using a Nicolet 6700 spectrometer (Thermo Fisher Scientific, USA) equipped with a liquid nitrogen-cooled MCT detector. A total of 512 scans were conducted for each spectrum. The spectra were collected over the temperature range from 30 to 100 °C. The FT-IR spectra of the 20 wt% solutions aged at 100 °C for 6, 12, 24 h and aged at 130 and 160 °C for 24 h were measured to assess the thermal degradation of PAN. The measurement of the all aged solutions was carried out at 30 °C. The ultraviolet–visible (UV–vis) spectra were recorded at 25 °C using Unicam 8700 series (Cambridge, UK) for the aged PAN solutions of 0.5 g/dl in DMF. The thermogravimetric analysis (TGA) of 20 wt% PAN solutions in DMF was carried out by Simultaneous TGA/DTA/DSC analyzer (SDT Q600, TA instruments, USA). The PAN solutions aged at 30, 60, and 90 °C for 24 h were used in the TGA measurement. Then, the TGA measurements of the aged PAN solutions was conducted over the temperature range from 25 to 400 °C at a rate of 10 °C/min under nitrogen atmosphere. The hydrodynamic diameter (Dh) of PAN in DMF was measured by a dynamic light scattering instrument (Zetasizer nano ZS, Malvern, UK) at 0.5 g/dl. The polymer solution was equilibrated at the designated temperature for 5 min prior to measurement. A He-Ne laser operating at 633 nm was used as a light source. The dynamic light scattering experiment was carried out at an angle of 90° to the incident beam and analyzed by regularized CONTIN method [29]. From the apparent diffusion coefficient, the hydrodynamic radius of the polymer was calculated by Stokes-Einstein equation. The intrinsic viscosity ([g]) of the fresh and aged PAN solutions was calculated by using the Huggins equation over the concentration range from 0.1 to 0.8 g/dl [30].
gred ¼
gsp c
0
¼ ½g þ kH ½g2 c
ð1Þ
in which, gred and gsp are reduced and specific viscosities measured by Ubbelohde viscometer (Schott Co., Germany), respectively. In addition, the kH and c stand for the Huggins constant and concentration, respectively. The [g] was obtained by extrapolating the gred values to c = 0 in Fig. S1, which assumes no intermolecular interactions. The rheological properties of 20 wt% PAN solutions were measured at 30 °C by Advanced Rheometric Expansion System (ARES, TA Instruments, USA). A parallel-plate geometry with a diameter of 40 mm was adopted. The plate gap and strain level were 1 mm and 5%, respectively. To prevent the evaporation of the solvent during measurement, a heavy mineral oil (Sigma-Aldrich, USA) was coated on the edge of the plates. The gelation behavior of the 20 wt% solutions was traced by conducting the rheological measurement at 30 °C as a function of the storage time at 25 °C up to 4 days. The PAN gels were prepared by conditioning the fresh and aged PAN solutions of 20 wt% at 25 °C for 14 days. Then, the large amplitude oscillatory shear (LAOS) behavior of the gels was evaluated. For the gel test, a sand paper coated parallelplate geometry with a diameter of 25 mm and plate gap of 3 mm were adopted, respectively. The LAOS measurement required both raw strain and stress signal which were acquired by means of the native control software (TA Orchestrator), using the arbitrary wave-shape test as suggested by Ewoldt et al. [31]. The arbitrary wave-shape test was performed at a frequency of 0.5 rad/s. In addition, the imposed strain amplitude (c0) ranged from 5 to 400%.
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3. Results and discussion 3.1. Conformational change of PAN during aging of the solutions Fig. 1 shows PAN solutions aged at 100, 130, and 160 °C and corresponding FT-IR spectra. The solution becomes yellowish with aging time at 100 °C but show little change in the IR spectra. On the other hand, aging at 130 and 160 °C for 24 h makes the solutions turbid and produces new bands in the spectra, whose assignments are summarized in Table 1. This structural change represents the thermal degradation of PAN via the cyclization of nitrile groups and unsaturation of the backbone [32–34]. Hence, the conformational change of PAN in DMF was evaluated below 100 °C in which little chemical reaction takes place. The conformation of PAN depends predominantly on the orientation modes of nitrile groups. The parallel orientation of nitrile groups corresponds to the planar zigzag structure, whereas the anti-parallel orientation indicates the helix one [21]. The presence of the physical association between nitrile groups can be verified by the UV–vis measurement, which offers information on the orientation mode of the groups. According to Andreyeva et al. [35], the isolated and physically associated nitrile groups produce the absorption peaks at 156 and 270 nm in the UV–vis spectra of the solutions, respectively. The fresh and aged PAN solutions of 0.5 g/dl give the absorption peak at 268 nm in Fig. S2, indicating the p-absorption of nitrile groups through the dipole-dipole pairs even in a single coil. The variation of the peak absorbance with aging temperature and time is plotted in Fig. 2. The absorbance is gradually increased with aging time above 60 °C, which is more noticeable at higher temperatures. This indicates the stronger p-absorption of nitrile groups by expanding the dipole-dipole pairs at higher temperatures. That is, nitrile groups are thermally reoriented to establish the suitable positions for the pairs by reducing their electrostatic repulsion. Such reorientation should induce the conformational change probably to the helix structure [13,14]. Then, the increased absorbance hardly recover to the initial value after cooling of the aged solutions. This well corresponds to the yellowing of the solutions after aging.
Fig. 1. The FT-IR spectra of 20 wt% PAN solutions in DMF, fresh (a) and aged at 100 °C for 6 (b), 12 (c), and 24 h (d), respectively. The optical micrograph of corresponding solutions were shown in the right column. The thermal degraded PAN solutions of 20 wt% aged at 130 (e) and 160 °C (f) for 24 h and their spectra were also displayed.
Table 1 The positions and assignments of the newly generated bands in the FT-IR spectra of the aged solutions at 130 and 160 °C for 24 h [32–34]. The band positions (cm1)
Assignments (Functional group)
3370 3210 2200 1620 1580 1250
NH2 NH C„N (b-aminonitrile) C@N, C@C mixed CAN, CAC mixed
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Fig. 2. The variation of the absorbance of the peak at 268 nm in the UV–vis spectra of 0.5 g/dl PAN solutions in DMF with aging time at a given temperature between 30 and 80 °C. The UV–vis measurement of all aged PAN solutions was carried out at 30 °C.
Fig. 3. The variations of the intrinsic viscosity ([g]) and hydrodynamic diameter (Dh) with aging temperature for 0.5 g/dl PAN solutions in DMF. The viscosity and DLS measurements of all aged PAN solutions were carried out at 30 °C.
The thermal-induced conformational change of PAN chains is clarified by the intrinsic viscosity ([g]) and hydrodynamic diameter (Dh), a measure of coil dimension of a polymer in a solvent. In Fig. 3, both [g] and Dh exhibit gradual decrease with aging temperature, which represents the reduction of the hydrodynamic volume of the individual PAN coils. Then, the extent of the decrease is much greater in the Dh than [g] because the intensity scattering of a polymer coil is proportional to the sixth power of its diameter. In other words, the slight change of the coil dimension is strongly amplified in the intensity size distribution by DLS. These results clearly confirm the conformational change of PAN from the extended structure with the planar-zigzag orientation of nitrile groups to the contracted one with the helix orientation. The latter structure is more stable than the former one due to low electrostatic repulsion of the adjacent nitrile groups although the helix backbone is distorted [21]. That is, the decrease of the hydrodynamic volume via the twisting of the chain produces more stable equilibrium state of the coil in DMF during aging at higher temperatures. In addition, the UV–vis result reveals that the development of the dipole-dipole pairs between nitrile groups contribute to the stabilization of the coil state under the considerable influence of the enthalpic effect. As a result, the chain conformation in the aged solutions hardly recover to the initial structure in the fresh one as illustrated in Scheme 1. The nitrile stretching (m(CN)) and carbonyl stretching (m(CO)) of DMF in the FT-IR spectra of PAN solutions over the temperature range from 30 to 100 °C are shown in Fig. 4. Both stretching bands obviously show the shifts with increasing temperature, suggesting the change of the dipole-dipole interactions between polymer and solvent molecules. The
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Scheme 1. The chain conformations and corresponding orientation of nitrile groups of PAN in the fresh and aged solutions in DMF.
temperature-dependent band shifts are clearly shown in Fig. 5. It is reported that both stretching bands are observed at lower frequencies when the stronger dipole-dipole interactions are formed [36–38]. That is, the lower frequencies of the m(CN) and m(CO) than those of the bulk state at 30 °C result from the dipole-dipole interactions between nitrile groups and DMF in the solutions. As temperature increases, however, the m(CO) exhibits the blue shift but m(CN) show the red shift. This indicates that the DMF molecules are dissociated and released from the polymer chains but nitrile groups produce stronger dipole-dipole interactions between each other at higher temperatures. These results agrees with the UV–vis results. The opposite trends of the band shifts can be clearly explained based on the chemical structures of PAN and DMF as illustrated in Scheme 2. The DMF molecule has the resonance structure, whereas nitrile groups are isolated by carbon-carbon single bond backbone. Moreover, the motion of the polymer molecules is largely restricted even at higher temperatures comparing to the solvent molecules. Hence, the polarization of the DMF molecules deteriorates rapidly as the entropy of the molecules is enhanced with increasing temperature, while nitrile groups retain relatively strong polarization state. As a result, the repulsion between the adjacent nitrile groups is enhanced, which leads to the conformational change of PAN to the helix-based structure during aging. In Fig. 5, moreover, the m(CO) at 60 °C is observed at a higher frequency than that of bulk DMF, which probably indicates the onset of the marked dissociation of the DMF molecules. This seems to accelerate the weakening of the dipole-dipole interactions between nitrile groups and DMF molecules and corresponding conformational change of PAN above 60 °C as evidenced by the UV–vis result. Fig. 6 shows TGA thermograms of PAN solutions with different aging temperature. TGA offers information on the change of the physical state of the DMF molecules after solution aging. The solutions aged at a higher temperature show faster evaporation of DMF up to 170 °C. However, the residual solvent above 170 °C, corresponding to the solvent molecules strongly bound with nitrile groups, evaporates more slowly in the solutions aged at a higher temperature. That is, the polarization of nitrile groups are too concentrated only on the bound solvent molecules in the aged solutions. This verifies that the conformational change and corresponding reorientation of nitrile groups cause the irregular interactions between PAN and DMF molecules. 3.2. The viscoelastic properties of PAN gels in the LAOS measurement The conformational change of the PAN chains and corresponding reorientation of nitrile groups would have significant effects on the rheological properties of the concentrated PAN solutions. Fig. 7a shows the dynamic viscosity (g0 ) curves of 20 wt% PAN solutions, fresh and aged at 100 °C (Ag100-solution). The Ag100-solution gives lower g0 than fresh one immediately after aging (Day-0). This indicates that the reduction of the hydrodynamic volume of the individual coils results in the decrease of the apparent molecular weight of PAN in the concentrated solutions. During gelation at 25 °C, however, the
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Fig. 4. The nitrile stretching of PAN (a) and carbonyl stretching of DMF (b) in the FT-IR spectra of 20 wt% PAN solutions over the temperature range from 30 to 100 °C, respectively.
Fig. 5. The variations of the positions of nitrile (m(CN)) and carbonyl (m(CO)) stretching bands for 20 wt% PAN solutions with increasing temperature. The m (CN) of PAN and m(CO) of DMF in the respective bulk states were obtained at 30 °C.
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Scheme 2. The charge distribution modes of PAN and DMF molecules and proposed mechanism of the change in the dipole-dipole interactions with temperature in PAN solutions.
Fig. 6. TGA thermograms of 20 wt% PAN solutions in DMF aged at 30, 60, and 90 °C. The TGA measurement of the aged PAN solutions was conducted over the temperature range from 25 to 400 °C at a rate of 10 °C/min under nitrogen atmosphere.
Ag100-solution shows more sharp increase of the g0 and noticeable viscoplastic behavior than fresh one. In addition, the storage modulus in Fig. 7b gives more plateau-like curve and greater value at a lower frequency region in the Ag100 system than in the fresh one, which confirms more rapid gelation behavior and stronger gel structure of the Ag100-solution. These results suggest that PAN gels prepared from the aged solution possess strong gel points through the tight entanglements between chains with the helix-based conformation. The phase change of the solutions during gelation is evaluated by the tand curves in Fig. 7c. An increase of the tand indicates a trend toward a liquid-like character, but the decrease indicates a trend toward a solid-like character [39–41]. Both PAN solutions show the frequency-dependent decrease of the tand, originating from the shear-induced chain orientation. However, the tand values are sharply decreased with storage time particularly at a lower frequency region, which is more
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Fig. 7. The dynamic viscosity (g0 ) (a), storage modulus (G0 ) (b), and loss tangent (tand) (c) curves of 20 wt% PAN solutions in DMF, fresh and aged at 100 °C (Ag100-solution). The PAN solutions were kept at 25 °C for 4 days for gelation and the rheological measurement was carried out at 30 °C.
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prominent with the Ag100-solution. That is, the enhanced intermolecular interactions between anti-parallel oriented nitrile groups in the helix PAN chains accelerates the gelation of the concentrated solutions. To assess the characteristics of the gel structure, the viscoelasticity of the PAN gels prepared from the fresh and aged solutions are explored in the LAOS measurement. Fig. 8 shows the stress waveform signals of PAN solutions and gels and corresponding Lissajous patterns obtained at the imposed strain amplitude (c0) of 400%. As a rule, the more ellipse-like shape of the elastic Lissajous curve indicates the higher elastic characters of the system, whose distortion (non-ellipse) represents the nonlinear viscoelasticity originating from the characteristic internal structure [42,43]. As expected, 20 wt% PAN solution gives the sinusoidal stress response with the circular shape Lissajous curve. The gel prepared from the fresh solution gives the ellipse-shape Lissajous curve because of the development of the gel network. However, the gel of the Ag100-solution (Ag100-gel) produces the distorted Lissajous curve and non-sinusoidal stress waveform. In addition, this unique shape of the Lissajous curve is reported to represent the strain-stiffening of the system, which probably results from the existence of the physical crosslinking points [44]. In the distorted Lissajous pattern, the shear stress is greater at large strains than one would expect by projecting the center portion of the ellipse. This reveals that the flow alignment of the polymer chains is restricted under the shear deformation. That is, the strain-stiffening is associated with the formation of complex internal structure and physical network. In consequence, PAN chains with more helix-like conformation in the aged solution produce more tough gel points, which strongly restricts the shear-induced chain orientation and extension. It gives rise to the strainstiffening characters of the gels. Fig. 9 exhibits the Lissajous curves of the PAN gels displayed in a Pipkin diagram according to the c0 and aging temperature of the raw solutions. As expected, the gels from the solutions aged above 60 °C noticeably exhibit the distorted Lissajous curves at a higher c0 above 100%. This verifies the obvious nonlinear viscoelasticity of the gels possessing the helix structure-based PAN chains. In Fig. 10 plotting the rmax of the individual Lissajous curves against aging temperature, the solution aging at higher temperatures above 60 °C produces the greater rmax of the gels. This reflects the increase of the gel strength. The thermoreversibility of the Ag100-gel and gels prepared from of the solutions aged at 60 (Ag60-gel) and 80 °C (Ag80-gel) is evaluated in the successive LAOS tests at 80 and 30 °C. Fig. 11 exhibits the change of the stress waveforms and Lissajous curves with time in the LAOS tests. Three PAN gels show the decrease of the stress amplitude with time at 80 °C, which means the breakdown of the gel structure by heat. At the same time, the Lissajous curves are transformed
Fig. 8. The waveform stress signals as a function of time and corresponding Lissajous patterns of 20 wt% fresh PAN solution (a) and gel (b). The gel prepared from the solution aged at 100 °C for 24 h (c) was evaluated as well. The LAOS measurement was conducted at 0.5 rad/s and 400% of frequency and imposed strain amplitude (c0), respectively.
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Fig. 9. The elastic Lissajous curves of the PAN gels displayed in a Pipkin diagram according to the imposed strain amplitude (c0) and aging temperature of the raw solutions of 20 wt%. The LAOS measurement was conducted at 30 °C.
Fig. 10. The variation of the maximum stress (rmax) in the Lissajous curves of the PAN gels with increasing aging temperature of the raw solutions of 20 wt %. The LAOS measurement was conducted at 30 °C.
to the less-distorted shapes. In addition, the Ag60 and Ag80-gels retain almost constant stress amplitude and identical Lissajous patterns in the second step at 30 °C. In Fig. 11c, however, the stress amplitude of the Ag100-gel considerably recovers to the initial value with time at 30 °C. This reveals the existence of the thermo-irreversible gel points. Moreover, the Lissajous curve gives more distorted shape than before at the end of the second step. The strain-stiffening behavior of the Ag100-gel can be quantitatively identified by using the strain-stiffening ratio (S) [45,46].
S ð%Þ ¼ ððGL GM Þ=GL Þ 100
ð2Þ
in which, the GM and GL represents the minimum and large-strain moduli obtained in the Lissajous curve as shown in the inset graph of Fig. 12. The greater value of the S indicates the stronger strain-stiffening behavior. Fig. 12 plots the variations of the rmax and S with time of the successive LAOS tests at 80 and 30 °C. Both parameters show the gradual decrease in the first step at 80 °C, followed by the increase in the second step at 30 °C. This restoration confirms the thermo-irreversible characters of the Ag100-gel. Consequently, strong gel points of PAN chains with helix conformation are hardly broken up
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Fig. 11. The waveform stress signals of the PAN gels prepared from the 20 wt% solutions respectively aged at 60 (a), 80 (b), and 100 °C (c) in the successive LAOS tests at 80 and 30 °C. The Lissajous curves obtained at a specific measurement time were displayed above the respective graphs. Between the two-step LAOS tests at 30 and 80 °C, the solutions were naturally cooled for 10 min. The LAOS measurement was conducted at 0.5 rad/s and 400% of frequency and imposed strain amplitude (c0), respectively.
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Fig. 12. The variations of the maximum stress (rmax) and strain-stiffening ratio (S) of the gel prepared from the solution aged at 100 °C with the number of cycles in the respective LAOS tests at 80 and 30 °C. Between the two-step LAOS tests at 30 and 80 °C, the solutions were naturally cooled for 10 min. The LAOS measurement was conducted at 0.5 rad/s and 400% of frequency and imposed strain amplitude (c0), respectively. The minimum (GM) and large (GL) strain moduli were schematically indicated in the inset graph.
by heat because the physical association of nitrile groups developed by aging at high temperature is thermally stable. Then, the S is much greater than the initial value. Thus, the gel structure mostly composed of the thermo-irreversible points shows very strong strain-stiffening behavior. 4. Conclusions The aging of PAN solutions above 60 °C induced the conformational change to the helix-based chain structure. This structural change was accompanied by the anti-parallel orientation of nitrile groups, which produced the dipole-dipole pairs. In addition, the conformational difference of PAN chains brought about the different nonlinear viscoelastic properties between the fresh and aged gels in the LAOS measurement. The helix-based conformation in the aged gel produced more tight entanglements between chains than in the fresh one by developing the higher content of the physical association of nitrile groups. The strong gel points in the aged systems were hardly broken up by heat as well as shear. As a result, the aged gel prepared from the solution aged at 100 °C showed strong strain-stiffening behavior and thermo-irreversible characters. Thus, it is prerequisite to understand the effects of the chain conformation on the physical properties of the solutions and gels for analyzing the type and degree of the internal structure and resulting phase change. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.eurpolymj.2016.10.037. References [1] R.B. Beevers, Temperature dependence of association in polyacrylonitrile solutions, Polymer 8 (9) (1967) 463–468. [2] S.J. Law, S.K. Mukhopadhyay, Investigation of wet-spun acrylic fiber morphology by membrane technology techniques, J. Appl. Polym. Sci. 62 (1) (1996) 33–47. [3] S.J. Law, S.K. Mukhopadhyay, The construction of a phase diagram for a ternary system used for the wet spinning of acrylic fibers based on a linearized cloudpoint curve correlation, J. Appl. Polym. Sci. 65 (11) (1997) 2131–2139. [4] A.F. Thunemann, W. Ruland, Lamellar mesophases in polyacrylonitrile: a synchrotron small-angle X-ray scattering study, Macromolecules 33 (7) (2000) 2626–2631. [5] J. Bisschops, Reversible gelation of concentrated polyacrylonitrile solutions, J. Polym. Sci. 12 (67) (1954) 583–597. [6] A. Malkin, S. Ilyin, T. Roumyantseva, V. Kulichikhin, Rheological evidence of gel formation in dilute poly(acrylonitrile) solutions, Macromolecules 46 (1) (2013) 257–266. [7] S. Morariu, C.E. Brunchi, C. Hulubei, M. Bercea, Influence of temperature on the rheological behavior of polymer mixtures in solution, Ind. Eng. Chem. Res. 50 (15) (2011) 9451–9455. [8] L. Xu, F. Qiu, Unusual viscosity behavior of polyacrylonitrile in NaSCN aqueous solutions, Polymer 64 (2015) 130–138. [9] L.J. Tan, S.P. Liu, D. Pan, Viscoelastic behavior of polyacrylonitrile/dimethyl sulfoxide concentrated solution during thermal-induced gelation, J. Phys. Chem. B 113 (3) (2009) 603–609. [10] L.J. Tan, S.P. Liu, D. Pan, N. Pan, Gelation of polyacrylonitrile in a mixed solvent: scaling and fractal analysis, Soft Matter 5 (21) (2009) 4297–4304. [11] L.J. Tan, D. Pan, N. Pan, Gelation behavior of polyacrylonitrile solution in relation to aging process and gel concentration, Polymer 49 (26) (2008) 5676– 5682. [12] L.J. Tan, A. Wan, Structural changes in thermal-induced polyacrylonitrile gel under uniaxial drawing, Colloid Surface. A 392 (1) (2011) 350–354.
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Effects of chain conformation on the viscoelastic properties of polyacrylonitrile gels under large amplitude oscillatory shear Youngho Eom, Byoung Chul Kim ⇑ Department of Organic and Nano Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea
a r t i c l e
i n f o
Article history: Received 18 September 2016 Received in revised form 17 October 2016 Accepted 21 October 2016 Available online 26 October 2016 Keywords: Polyacrylonitrile gel Conformational change Aging Viscoelasticity Large amplitude oscillatory shear
a b s t r a c t The viscoelastic properties of polyacrylonitrile (PAN) gels prepared from the 20 wt% solutions in N,N-dimethyl formamide (DMF) were investigated under large amplitude oscillatory shear (LAOS) in terms of aging temperature of the solutions from 30 to 100 °C. Aging of PAN solutions reduced intrinsic viscosity and hydrodynamic diameter with little degradation. In the UV–vis spectra, the absorption peak at 268 nm represented the dipoledipole pairs of physically associated nitrile groups, whose intensity increased with aging temperature. These results confirmed that aging of the solutions gave rise to the contracted chain conformation with anti-parallel orientation-dominant nitrile groups. In 20 wt%, aged solutions showed gelation behavior more rapid than fresh one during storage at 25 °C. At the imposed strain amplitude of 400% in the LAOS measurement, PAN gel prepared from fresh solution gave sinusoidal waveform signal of stress and ellipse-shape Lissajous curve. However, gels from the solutions aged above 60 °C showed non-sinusoidal stress waveform and characteristic Lissajous pattern, representing strain-stiffening behavior. In addition, the gel of the solution aged at 100 °C gave the greater strain-stiffening ratio than the initial value after the successive LAOS tests at 80 and 30 °C, indicating the thermo-irreversibility. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Polyacrylonitrile (PAN) is one of the most studied polymers owing to the highly polar chemical structure as well as various applications such as apparel and industrial fibers [1–4]. As well recognized, the high polarity of nitrile groups makes the polymer infusible in the bulk state and resulting dipole-dipole interactions in the solutions bring about the complicated phase change and gelation with processing and environmental conditions [5–12]. In fact, the intermolecular interactions between nitrile groups are profoundly affected by their orientation modes depending on the chain conformation [13,14]. Hence, the chain conformation and corresponding orientation of nitrile groups have been explored both theoretically and empirically by using the computational simulation of the model structure, NMR, FT-IR, and X-ray techniques in the solid state, and light scattering and viscosity measurements in the solution state, respectively [15–23]. In theory, PAN chain produces the planar-zigzag or helix structure according to the nitrile group orientation but gives the irregular helix structure in practice [21]. The correlations between the chain conformation and structural characters of PAN fibers have been extensively investigated to obtain the desirable mechanical properties [24–27]. In comparison with the bulk PAN and its fibers, little literature on the effects of the chain conformation on the physical properties of the solutions and gels is available because ⇑ Corresponding author. E-mail address: [email protected] (B.C. Kim). http://dx.doi.org/10.1016/j.eurpolymj.2016.10.037 0014-3057/Ó 2016 Elsevier Ltd. All rights reserved.
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dipole-dipole interactions between polymer and solvent give rise to very complicated polymer structure. However, understanding the dependence of the solution and gel properties on the polymer structure offers critical information on the analysis of the phase changes of the systems due to the direct correlation between the conformation and interaction mode of nitrile groups. The conformational change of PAN in N,N-dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) was explored with respect to the solubility and temperature change [28]. In this study, we found that PAN gave different chain conformations in the fresh and aged solutions and corresponding gels. Moreover, such structural difference had significant effects on the rheological properties particularly for the gels. That is, the chain conformation would be closely related to the entanglement behavior and gel structure. Hence, the viscoelastic properties of PAN gels prepared from the 20 wt% solutions with different aging temperature ranging from 30 to 100 °C were assessed using the large amplitude oscillatory shear flow. 2. Experimental 2.1. Materials PAN was purchased from Sigma-Aldrich (USA), whose weight average molecular weight was 150,000. EP grade of DMF was purchased from Duksan Co. (Korea). PAN and DMF were used without further purification. PAN was vacuum dried at 60 °C for 24 h and dissolved in the solvents with stirring at 60 °C for 3 h until the solution became optically transparent. Before solution aging, all PAN solutions were purged with nitrogen at 25 °C for 15 min to minimize the oxidative degradation. The nitrogen purged solutions were aged over the temperature and time ranges from 30 to 160 °C and 5 min to 24 h, respectively. 2.2. Measurement of physical properties The FT-IR spectra of PAN solutions in DMF were obtained by using 20-times reflectance ATR-FTIR (Ge crystal and incident angle of 45°) with heating controller. All temperature-dependent spectra were recorded with a spectral resolution of 4 cm1 using a Nicolet 6700 spectrometer (Thermo Fisher Scientific, USA) equipped with a liquid nitrogen-cooled MCT detector. A total of 512 scans were conducted for each spectrum. The spectra were collected over the temperature range from 30 to 100 °C. The FT-IR spectra of the 20 wt% solutions aged at 100 °C for 6, 12, 24 h and aged at 130 and 160 °C for 24 h were measured to assess the thermal degradation of PAN. The measurement of the all aged solutions was carried out at 30 °C. The ultraviolet–visible (UV–vis) spectra were recorded at 25 °C using Unicam 8700 series (Cambridge, UK) for the aged PAN solutions of 0.5 g/dl in DMF. The thermogravimetric analysis (TGA) of 20 wt% PAN solutions in DMF was carried out by Simultaneous TGA/DTA/DSC analyzer (SDT Q600, TA instruments, USA). The PAN solutions aged at 30, 60, and 90 °C for 24 h were used in the TGA measurement. Then, the TGA measurements of the aged PAN solutions was conducted over the temperature range from 25 to 400 °C at a rate of 10 °C/min under nitrogen atmosphere. The hydrodynamic diameter (Dh) of PAN in DMF was measured by a dynamic light scattering instrument (Zetasizer nano ZS, Malvern, UK) at 0.5 g/dl. The polymer solution was equilibrated at the designated temperature for 5 min prior to measurement. A He-Ne laser operating at 633 nm was used as a light source. The dynamic light scattering experiment was carried out at an angle of 90° to the incident beam and analyzed by regularized CONTIN method [29]. From the apparent diffusion coefficient, the hydrodynamic radius of the polymer was calculated by Stokes-Einstein equation. The intrinsic viscosity ([g]) of the fresh and aged PAN solutions was calculated by using the Huggins equation over the concentration range from 0.1 to 0.8 g/dl [30].
gred ¼
gsp c
0
¼ ½g þ kH ½g2 c
ð1Þ
in which, gred and gsp are reduced and specific viscosities measured by Ubbelohde viscometer (Schott Co., Germany), respectively. In addition, the kH and c stand for the Huggins constant and concentration, respectively. The [g] was obtained by extrapolating the gred values to c = 0 in Fig. S1, which assumes no intermolecular interactions. The rheological properties of 20 wt% PAN solutions were measured at 30 °C by Advanced Rheometric Expansion System (ARES, TA Instruments, USA). A parallel-plate geometry with a diameter of 40 mm was adopted. The plate gap and strain level were 1 mm and 5%, respectively. To prevent the evaporation of the solvent during measurement, a heavy mineral oil (Sigma-Aldrich, USA) was coated on the edge of the plates. The gelation behavior of the 20 wt% solutions was traced by conducting the rheological measurement at 30 °C as a function of the storage time at 25 °C up to 4 days. The PAN gels were prepared by conditioning the fresh and aged PAN solutions of 20 wt% at 25 °C for 14 days. Then, the large amplitude oscillatory shear (LAOS) behavior of the gels was evaluated. For the gel test, a sand paper coated parallelplate geometry with a diameter of 25 mm and plate gap of 3 mm were adopted, respectively. The LAOS measurement required both raw strain and stress signal which were acquired by means of the native control software (TA Orchestrator), using the arbitrary wave-shape test as suggested by Ewoldt et al. [31]. The arbitrary wave-shape test was performed at a frequency of 0.5 rad/s. In addition, the imposed strain amplitude (c0) ranged from 5 to 400%.
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3. Results and discussion 3.1. Conformational change of PAN during aging of the solutions Fig. 1 shows PAN solutions aged at 100, 130, and 160 °C and corresponding FT-IR spectra. The solution becomes yellowish with aging time at 100 °C but show little change in the IR spectra. On the other hand, aging at 130 and 160 °C for 24 h makes the solutions turbid and produces new bands in the spectra, whose assignments are summarized in Table 1. This structural change represents the thermal degradation of PAN via the cyclization of nitrile groups and unsaturation of the backbone [32–34]. Hence, the conformational change of PAN in DMF was evaluated below 100 °C in which little chemical reaction takes place. The conformation of PAN depends predominantly on the orientation modes of nitrile groups. The parallel orientation of nitrile groups corresponds to the planar zigzag structure, whereas the anti-parallel orientation indicates the helix one [21]. The presence of the physical association between nitrile groups can be verified by the UV–vis measurement, which offers information on the orientation mode of the groups. According to Andreyeva et al. [35], the isolated and physically associated nitrile groups produce the absorption peaks at 156 and 270 nm in the UV–vis spectra of the solutions, respectively. The fresh and aged PAN solutions of 0.5 g/dl give the absorption peak at 268 nm in Fig. S2, indicating the p-absorption of nitrile groups through the dipole-dipole pairs even in a single coil. The variation of the peak absorbance with aging temperature and time is plotted in Fig. 2. The absorbance is gradually increased with aging time above 60 °C, which is more noticeable at higher temperatures. This indicates the stronger p-absorption of nitrile groups by expanding the dipole-dipole pairs at higher temperatures. That is, nitrile groups are thermally reoriented to establish the suitable positions for the pairs by reducing their electrostatic repulsion. Such reorientation should induce the conformational change probably to the helix structure [13,14]. Then, the increased absorbance hardly recover to the initial value after cooling of the aged solutions. This well corresponds to the yellowing of the solutions after aging.
Fig. 1. The FT-IR spectra of 20 wt% PAN solutions in DMF, fresh (a) and aged at 100 °C for 6 (b), 12 (c), and 24 h (d), respectively. The optical micrograph of corresponding solutions were shown in the right column. The thermal degraded PAN solutions of 20 wt% aged at 130 (e) and 160 °C (f) for 24 h and their spectra were also displayed.
Table 1 The positions and assignments of the newly generated bands in the FT-IR spectra of the aged solutions at 130 and 160 °C for 24 h [32–34]. The band positions (cm1)
Assignments (Functional group)
3370 3210 2200 1620 1580 1250
NH2 NH C„N (b-aminonitrile) C@N, C@C mixed CAN, CAC mixed
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Fig. 2. The variation of the absorbance of the peak at 268 nm in the UV–vis spectra of 0.5 g/dl PAN solutions in DMF with aging time at a given temperature between 30 and 80 °C. The UV–vis measurement of all aged PAN solutions was carried out at 30 °C.
Fig. 3. The variations of the intrinsic viscosity ([g]) and hydrodynamic diameter (Dh) with aging temperature for 0.5 g/dl PAN solutions in DMF. The viscosity and DLS measurements of all aged PAN solutions were carried out at 30 °C.
The thermal-induced conformational change of PAN chains is clarified by the intrinsic viscosity ([g]) and hydrodynamic diameter (Dh), a measure of coil dimension of a polymer in a solvent. In Fig. 3, both [g] and Dh exhibit gradual decrease with aging temperature, which represents the reduction of the hydrodynamic volume of the individual PAN coils. Then, the extent of the decrease is much greater in the Dh than [g] because the intensity scattering of a polymer coil is proportional to the sixth power of its diameter. In other words, the slight change of the coil dimension is strongly amplified in the intensity size distribution by DLS. These results clearly confirm the conformational change of PAN from the extended structure with the planar-zigzag orientation of nitrile groups to the contracted one with the helix orientation. The latter structure is more stable than the former one due to low electrostatic repulsion of the adjacent nitrile groups although the helix backbone is distorted [21]. That is, the decrease of the hydrodynamic volume via the twisting of the chain produces more stable equilibrium state of the coil in DMF during aging at higher temperatures. In addition, the UV–vis result reveals that the development of the dipole-dipole pairs between nitrile groups contribute to the stabilization of the coil state under the considerable influence of the enthalpic effect. As a result, the chain conformation in the aged solutions hardly recover to the initial structure in the fresh one as illustrated in Scheme 1. The nitrile stretching (m(CN)) and carbonyl stretching (m(CO)) of DMF in the FT-IR spectra of PAN solutions over the temperature range from 30 to 100 °C are shown in Fig. 4. Both stretching bands obviously show the shifts with increasing temperature, suggesting the change of the dipole-dipole interactions between polymer and solvent molecules. The
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Scheme 1. The chain conformations and corresponding orientation of nitrile groups of PAN in the fresh and aged solutions in DMF.
temperature-dependent band shifts are clearly shown in Fig. 5. It is reported that both stretching bands are observed at lower frequencies when the stronger dipole-dipole interactions are formed [36–38]. That is, the lower frequencies of the m(CN) and m(CO) than those of the bulk state at 30 °C result from the dipole-dipole interactions between nitrile groups and DMF in the solutions. As temperature increases, however, the m(CO) exhibits the blue shift but m(CN) show the red shift. This indicates that the DMF molecules are dissociated and released from the polymer chains but nitrile groups produce stronger dipole-dipole interactions between each other at higher temperatures. These results agrees with the UV–vis results. The opposite trends of the band shifts can be clearly explained based on the chemical structures of PAN and DMF as illustrated in Scheme 2. The DMF molecule has the resonance structure, whereas nitrile groups are isolated by carbon-carbon single bond backbone. Moreover, the motion of the polymer molecules is largely restricted even at higher temperatures comparing to the solvent molecules. Hence, the polarization of the DMF molecules deteriorates rapidly as the entropy of the molecules is enhanced with increasing temperature, while nitrile groups retain relatively strong polarization state. As a result, the repulsion between the adjacent nitrile groups is enhanced, which leads to the conformational change of PAN to the helix-based structure during aging. In Fig. 5, moreover, the m(CO) at 60 °C is observed at a higher frequency than that of bulk DMF, which probably indicates the onset of the marked dissociation of the DMF molecules. This seems to accelerate the weakening of the dipole-dipole interactions between nitrile groups and DMF molecules and corresponding conformational change of PAN above 60 °C as evidenced by the UV–vis result. Fig. 6 shows TGA thermograms of PAN solutions with different aging temperature. TGA offers information on the change of the physical state of the DMF molecules after solution aging. The solutions aged at a higher temperature show faster evaporation of DMF up to 170 °C. However, the residual solvent above 170 °C, corresponding to the solvent molecules strongly bound with nitrile groups, evaporates more slowly in the solutions aged at a higher temperature. That is, the polarization of nitrile groups are too concentrated only on the bound solvent molecules in the aged solutions. This verifies that the conformational change and corresponding reorientation of nitrile groups cause the irregular interactions between PAN and DMF molecules. 3.2. The viscoelastic properties of PAN gels in the LAOS measurement The conformational change of the PAN chains and corresponding reorientation of nitrile groups would have significant effects on the rheological properties of the concentrated PAN solutions. Fig. 7a shows the dynamic viscosity (g0 ) curves of 20 wt% PAN solutions, fresh and aged at 100 °C (Ag100-solution). The Ag100-solution gives lower g0 than fresh one immediately after aging (Day-0). This indicates that the reduction of the hydrodynamic volume of the individual coils results in the decrease of the apparent molecular weight of PAN in the concentrated solutions. During gelation at 25 °C, however, the
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Fig. 4. The nitrile stretching of PAN (a) and carbonyl stretching of DMF (b) in the FT-IR spectra of 20 wt% PAN solutions over the temperature range from 30 to 100 °C, respectively.
Fig. 5. The variations of the positions of nitrile (m(CN)) and carbonyl (m(CO)) stretching bands for 20 wt% PAN solutions with increasing temperature. The m (CN) of PAN and m(CO) of DMF in the respective bulk states were obtained at 30 °C.
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Scheme 2. The charge distribution modes of PAN and DMF molecules and proposed mechanism of the change in the dipole-dipole interactions with temperature in PAN solutions.
Fig. 6. TGA thermograms of 20 wt% PAN solutions in DMF aged at 30, 60, and 90 °C. The TGA measurement of the aged PAN solutions was conducted over the temperature range from 25 to 400 °C at a rate of 10 °C/min under nitrogen atmosphere.
Ag100-solution shows more sharp increase of the g0 and noticeable viscoplastic behavior than fresh one. In addition, the storage modulus in Fig. 7b gives more plateau-like curve and greater value at a lower frequency region in the Ag100 system than in the fresh one, which confirms more rapid gelation behavior and stronger gel structure of the Ag100-solution. These results suggest that PAN gels prepared from the aged solution possess strong gel points through the tight entanglements between chains with the helix-based conformation. The phase change of the solutions during gelation is evaluated by the tand curves in Fig. 7c. An increase of the tand indicates a trend toward a liquid-like character, but the decrease indicates a trend toward a solid-like character [39–41]. Both PAN solutions show the frequency-dependent decrease of the tand, originating from the shear-induced chain orientation. However, the tand values are sharply decreased with storage time particularly at a lower frequency region, which is more
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Fig. 7. The dynamic viscosity (g0 ) (a), storage modulus (G0 ) (b), and loss tangent (tand) (c) curves of 20 wt% PAN solutions in DMF, fresh and aged at 100 °C (Ag100-solution). The PAN solutions were kept at 25 °C for 4 days for gelation and the rheological measurement was carried out at 30 °C.
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prominent with the Ag100-solution. That is, the enhanced intermolecular interactions between anti-parallel oriented nitrile groups in the helix PAN chains accelerates the gelation of the concentrated solutions. To assess the characteristics of the gel structure, the viscoelasticity of the PAN gels prepared from the fresh and aged solutions are explored in the LAOS measurement. Fig. 8 shows the stress waveform signals of PAN solutions and gels and corresponding Lissajous patterns obtained at the imposed strain amplitude (c0) of 400%. As a rule, the more ellipse-like shape of the elastic Lissajous curve indicates the higher elastic characters of the system, whose distortion (non-ellipse) represents the nonlinear viscoelasticity originating from the characteristic internal structure [42,43]. As expected, 20 wt% PAN solution gives the sinusoidal stress response with the circular shape Lissajous curve. The gel prepared from the fresh solution gives the ellipse-shape Lissajous curve because of the development of the gel network. However, the gel of the Ag100-solution (Ag100-gel) produces the distorted Lissajous curve and non-sinusoidal stress waveform. In addition, this unique shape of the Lissajous curve is reported to represent the strain-stiffening of the system, which probably results from the existence of the physical crosslinking points [44]. In the distorted Lissajous pattern, the shear stress is greater at large strains than one would expect by projecting the center portion of the ellipse. This reveals that the flow alignment of the polymer chains is restricted under the shear deformation. That is, the strain-stiffening is associated with the formation of complex internal structure and physical network. In consequence, PAN chains with more helix-like conformation in the aged solution produce more tough gel points, which strongly restricts the shear-induced chain orientation and extension. It gives rise to the strainstiffening characters of the gels. Fig. 9 exhibits the Lissajous curves of the PAN gels displayed in a Pipkin diagram according to the c0 and aging temperature of the raw solutions. As expected, the gels from the solutions aged above 60 °C noticeably exhibit the distorted Lissajous curves at a higher c0 above 100%. This verifies the obvious nonlinear viscoelasticity of the gels possessing the helix structure-based PAN chains. In Fig. 10 plotting the rmax of the individual Lissajous curves against aging temperature, the solution aging at higher temperatures above 60 °C produces the greater rmax of the gels. This reflects the increase of the gel strength. The thermoreversibility of the Ag100-gel and gels prepared from of the solutions aged at 60 (Ag60-gel) and 80 °C (Ag80-gel) is evaluated in the successive LAOS tests at 80 and 30 °C. Fig. 11 exhibits the change of the stress waveforms and Lissajous curves with time in the LAOS tests. Three PAN gels show the decrease of the stress amplitude with time at 80 °C, which means the breakdown of the gel structure by heat. At the same time, the Lissajous curves are transformed
Fig. 8. The waveform stress signals as a function of time and corresponding Lissajous patterns of 20 wt% fresh PAN solution (a) and gel (b). The gel prepared from the solution aged at 100 °C for 24 h (c) was evaluated as well. The LAOS measurement was conducted at 0.5 rad/s and 400% of frequency and imposed strain amplitude (c0), respectively.
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Fig. 9. The elastic Lissajous curves of the PAN gels displayed in a Pipkin diagram according to the imposed strain amplitude (c0) and aging temperature of the raw solutions of 20 wt%. The LAOS measurement was conducted at 30 °C.
Fig. 10. The variation of the maximum stress (rmax) in the Lissajous curves of the PAN gels with increasing aging temperature of the raw solutions of 20 wt %. The LAOS measurement was conducted at 30 °C.
to the less-distorted shapes. In addition, the Ag60 and Ag80-gels retain almost constant stress amplitude and identical Lissajous patterns in the second step at 30 °C. In Fig. 11c, however, the stress amplitude of the Ag100-gel considerably recovers to the initial value with time at 30 °C. This reveals the existence of the thermo-irreversible gel points. Moreover, the Lissajous curve gives more distorted shape than before at the end of the second step. The strain-stiffening behavior of the Ag100-gel can be quantitatively identified by using the strain-stiffening ratio (S) [45,46].
S ð%Þ ¼ ððGL GM Þ=GL Þ 100
ð2Þ
in which, the GM and GL represents the minimum and large-strain moduli obtained in the Lissajous curve as shown in the inset graph of Fig. 12. The greater value of the S indicates the stronger strain-stiffening behavior. Fig. 12 plots the variations of the rmax and S with time of the successive LAOS tests at 80 and 30 °C. Both parameters show the gradual decrease in the first step at 80 °C, followed by the increase in the second step at 30 °C. This restoration confirms the thermo-irreversible characters of the Ag100-gel. Consequently, strong gel points of PAN chains with helix conformation are hardly broken up
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Fig. 11. The waveform stress signals of the PAN gels prepared from the 20 wt% solutions respectively aged at 60 (a), 80 (b), and 100 °C (c) in the successive LAOS tests at 80 and 30 °C. The Lissajous curves obtained at a specific measurement time were displayed above the respective graphs. Between the two-step LAOS tests at 30 and 80 °C, the solutions were naturally cooled for 10 min. The LAOS measurement was conducted at 0.5 rad/s and 400% of frequency and imposed strain amplitude (c0), respectively.
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Fig. 12. The variations of the maximum stress (rmax) and strain-stiffening ratio (S) of the gel prepared from the solution aged at 100 °C with the number of cycles in the respective LAOS tests at 80 and 30 °C. Between the two-step LAOS tests at 30 and 80 °C, the solutions were naturally cooled for 10 min. The LAOS measurement was conducted at 0.5 rad/s and 400% of frequency and imposed strain amplitude (c0), respectively. The minimum (GM) and large (GL) strain moduli were schematically indicated in the inset graph.
by heat because the physical association of nitrile groups developed by aging at high temperature is thermally stable. Then, the S is much greater than the initial value. Thus, the gel structure mostly composed of the thermo-irreversible points shows very strong strain-stiffening behavior. 4. Conclusions The aging of PAN solutions above 60 °C induced the conformational change to the helix-based chain structure. This structural change was accompanied by the anti-parallel orientation of nitrile groups, which produced the dipole-dipole pairs. In addition, the conformational difference of PAN chains brought about the different nonlinear viscoelastic properties between the fresh and aged gels in the LAOS measurement. The helix-based conformation in the aged gel produced more tight entanglements between chains than in the fresh one by developing the higher content of the physical association of nitrile groups. The strong gel points in the aged systems were hardly broken up by heat as well as shear. As a result, the aged gel prepared from the solution aged at 100 °C showed strong strain-stiffening behavior and thermo-irreversible characters. Thus, it is prerequisite to understand the effects of the chain conformation on the physical properties of the solutions and gels for analyzing the type and degree of the internal structure and resulting phase change. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.eurpolymj.2016.10.037. References [1] R.B. Beevers, Temperature dependence of association in polyacrylonitrile solutions, Polymer 8 (9) (1967) 463–468. [2] S.J. Law, S.K. Mukhopadhyay, Investigation of wet-spun acrylic fiber morphology by membrane technology techniques, J. Appl. Polym. Sci. 62 (1) (1996) 33–47. [3] S.J. Law, S.K. Mukhopadhyay, The construction of a phase diagram for a ternary system used for the wet spinning of acrylic fibers based on a linearized cloudpoint curve correlation, J. Appl. Polym. Sci. 65 (11) (1997) 2131–2139. [4] A.F. Thunemann, W. Ruland, Lamellar mesophases in polyacrylonitrile: a synchrotron small-angle X-ray scattering study, Macromolecules 33 (7) (2000) 2626–2631. [5] J. Bisschops, Reversible gelation of concentrated polyacrylonitrile solutions, J. Polym. Sci. 12 (67) (1954) 583–597. [6] A. Malkin, S. Ilyin, T. Roumyantseva, V. Kulichikhin, Rheological evidence of gel formation in dilute poly(acrylonitrile) solutions, Macromolecules 46 (1) (2013) 257–266. [7] S. Morariu, C.E. Brunchi, C. Hulubei, M. Bercea, Influence of temperature on the rheological behavior of polymer mixtures in solution, Ind. Eng. Chem. Res. 50 (15) (2011) 9451–9455. [8] L. Xu, F. Qiu, Unusual viscosity behavior of polyacrylonitrile in NaSCN aqueous solutions, Polymer 64 (2015) 130–138. [9] L.J. Tan, S.P. Liu, D. Pan, Viscoelastic behavior of polyacrylonitrile/dimethyl sulfoxide concentrated solution during thermal-induced gelation, J. Phys. Chem. B 113 (3) (2009) 603–609. [10] L.J. Tan, S.P. Liu, D. Pan, N. Pan, Gelation of polyacrylonitrile in a mixed solvent: scaling and fractal analysis, Soft Matter 5 (21) (2009) 4297–4304. [11] L.J. Tan, D. Pan, N. Pan, Gelation behavior of polyacrylonitrile solution in relation to aging process and gel concentration, Polymer 49 (26) (2008) 5676– 5682. [12] L.J. Tan, A. Wan, Structural changes in thermal-induced polyacrylonitrile gel under uniaxial drawing, Colloid Surface. A 392 (1) (2011) 350–354.
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