Yu Wangab,
Zefan Wangb,
Ping Zhub,
Xinran Liubc,
Lei Wang*a,
Xia Dong*bc and
Dujin Wangbc
aShenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, P. R. China. E-mail: wl@szu.edu.cn
bCAS Key Laboratory of Engineering Plastics, Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. E-mail: xiadong@iccas.ac.cn
cUniversity of Chinese Academy of Science, Beijing, 100049, P. R. China
First published on 10th February 2021
The temperature dependence of the rheological properties of poly(ether-b-amide) (PEBA) segmented copolymer under oscillatory shear flow has been investigated. The magnitude of the dynamic storage modulus is affected by the physical microphase separation and irreversible crosslinking network, with the latter spontaneously forming between the polyamide segments and becoming the dominant factor in determining the microstructural evolution at temperatures well above the melting point of PEBA. From the rheological results, the initial temperature of the rheological properties dominated by the microphase separation and crosslinking (Tcross) structures were determined, respectively. Based on the two obtained temperatures, the microstructure evolution upon the heating can be separated into the ternary microstructure domains: homogenous (temperature below ), microphase separation dominating (between and Tcross), and crosslinking dominating domains (above Tcross). When the PEBA is heated to above Tcross, the content of crosslinking network increases with time and temperature, leading to an irreversible and non-negligible influence on the rheological, crystallization, and mechanical properties. A more pronounced strain-hardening phenomenon during the uniaxial stretching is observed for the sample with a higher content of crosslinking network.
For the segmented copolymers such as thermoplastic polyurethane (TPU),12,20 poly(ether-b-amide) (PEBA),28 poly(ester-ether) (PEE),14 olefin block copolymers (OBC),29 and transparent polyamide random copolymers,30 the structure–property relationship has been widely explored in the past three decades.7,10,30–32 Velankar and Cooper carried out a series of work to study the influence of chain architectures (including block length and block species) on the microphase separation of TPU,12,13,33 which provides an in-depth understanding on the phase behavior of segmented copolymers. The influence of microphase separation on the crystallization of segmented copolymers at quiescent or flow states also has been investigated by various techniques.28,30,34–36 It was found that the microphase separation at molten state has an effect on the morphology and size of the hard domain's crystals. Due to the co-existence nature of multi-components and multi-phase, the microstructure evolution of semi-crystalline segmented copolymer during the deformation is complicated. As the applied strain increased, the segmental chains' orientation, the crystal–crystal transition, and strain-induced crystallization can occur.37–40 Studying the microstructural evolution under the external fields is important for a better understanding of the macroscopic properties of the segmented copolymers. To date, the phase diagram of multi-block copolymers cannot be predicated well due to the more complex segmental interactions. In other words, the phase transition temperatures and the corresponding morphologies can only be determined experimentally.
PEBA segmented copolymer, which contains the polyamide hard-segment and polyether soft-segment, has been widely used in various fields such as sports equipment, polar clothing, and medical supplies. However, the studies focused on the temperature effect on the rheological properties,41 and crystallization morphology are limited.35 The systematic explore the microstructure evolution upon temperature is inadequate. It was reported that the polyamide crosslinking reaction could occur at the molten state in various polyamide (PA) systems, including PA6,42,43 PA6,6,42–45 PA6,10,42,43 PA11,46 PA12,47 and PA1012,48 especially after prolonged heating. The polyamide crosslinking is caused by the formation of the secondary amine groups,49 which reacts further with carboxyl groups to form the branched structures at high temperature.50 Therefore, the influence of the formed crosslinking structure on the microstructure evolution should not be ignored for the PEBA segmented copolymers.
The present investigation aims to probe the temperature dependence on the microstructure from apparent homogenous to crosslinked states via the small amplitude oscillatory shear (SAOS) measurement for the commercial PEBA copolymer with the trade name of Pebax®, for which contains the crystalline hard segment of PA12, while poly(tetramethylene oxide) (PTMO) as the soft segment.28,51 Based on the experimental results, there different microstructure domains of apparent homogenous, microphase separation, and crosslinking structure can be quantitatively determined. The influences of the microstructure on the rheological and mechanical properties is also studied by the thermal history under different temperature domains.
Fig. 1 General chemical structure of the studied Pebax® elastomers. The repeating numbers of PA12 HS and PTMO SS are denoted as “x” and “y”, respectively. |
Sample | WPA | Tc,SS (°C) | Tm,SS (°C) | Tc,HS (°C) | ΔHc,HS (J g−1) | Tm,HS (°C) | ΔHm,HS (J g−1) | Xc | |
---|---|---|---|---|---|---|---|---|---|
a The raw pellet of the PEBA elastomers were used as received.b The films were quenched from 270 °C after the 2 hours isothermal process. | |||||||||
P25D | Raw pelleta | 0.27 | −10.2 | 12.0 | 53.8 | 9.3 | 135.2 | 10.0 | 0.15 |
Treated filmb | −10.4 | 12.1 | 50.2 | 6.0 | 135.4 | 2.4 | 0.04 | ||
P35D | Raw pelleta | 0.29 | −11.9 | 9.0 | 68.9 | 14.5 | 145.0 | 14.7 | 0.21 |
Treated filmb | −10.7 | 9.2 | 68.5 | 8.9 | 145.2 | 5.9 | 0.08 | ||
P40D | Raw pelleta | 0.46 | −13.0 | 6.8 | 90.0 | 26.2 | 160.5 | 25.6 | 0.23 |
Treated filmb | −12.0 | 6.0 | 89.0 | 25.7 | 158.4 | 19.5 | 0.17 |
The thermal properties of the three studied copolymers were determined by differential scanning calorimetry (DSC Q2000, TA instruments). The instrument was calibrated with indium before measurements. Temperature scans were performed in the range of −50 to 200 °C with a heating/cooling rate of 10 °C min−1 under the nitrogen atmosphere. The melting temperature (Tm), crystallization temperature (Tc), the corresponding melting and crystallization enthalpies (ΔHm and ΔHc) for the PA12 and PTMO crystals are denoted by the subscript “HS” and “SS”, respectively. The crystallinity of PA12 crystals (Xc) was calculated by the normalized ΔHm,HS with the fraction of PA12 HS dividing by the fusion enthalpy of perfect PA12 crystals (246 J g−1).40 The results are listed in Table 1.
To remove the absorbed moisture, the raw pellets were dried at 100 °C under vacuum for 12 hours. Both the thin films with a thickness of 0.5 mm and the rheological specimens with 25 mm diameter and a thickness of 1 mm were prepared by melt-pressed under 180 °C and 50 MPa for 3 min by mold, then quickly cooled to room temperature by cold compression for 3 min.
The samples were heated to different isothermal temperatures (Tiso = 170, 190, 200, 210, 230, 250, and 270 °C), kept for 3 min to ensure all the crystals were melted, then the dynamic time sweep was performed for obtaining the time dependence of the dynamic storage modulus (G′) and loss modulus (G′′) within 2 hours isothermal at each temperature. To understand the time effect on the phase state, the G′ and G′′ over a frequency (ω) range of 0.1–628 rad s−1 were measured by the SAOS mode for each sample before and after the 2 hours isothermal processes, respectively.
In order to determine the microphase separation transition temperature of P35D melt, the dynamic temperature ramp measurement at a given ω of 0.5 rad s−1 with various heating rates of 0.5, 1.0, 1.5, and 2.0 °C min−1 from 160 to 270 °C were carried out by hot nitrogen gas to control the temperature in the accuracy of ±0.1 °C. With the obtained G′ and G′′, the binodal (Tb) and spinodal temperature (Ts) can be determined based on the Ajji and Chopin's model.54–56
The measurement was repeated 3 times for each condition.
Fig. 2 Double Logarithmic plots of G′ against ω (a) before and (b) after the 2 hours isothermal treatment at different Tiso. |
FTIR result provides another piece of evidence for the existence of crosslinking structure. Fig. 3 shows the two FTIR spectra of the P35D hot-pressed films with and without the 2 hours isothermal treatment at 270 °C. It was found that the band position and relative height of the bands related to amide group between the two spectra are different, i.e., the crosslinking network is closely associated with the amide groups. With the crosslinking network, the amide A band (stretching band of N–H group) at 3295 cm−1 shifts toward higher wavenumbers, indicating weakening strength of the hydrogen bonding. Moreover, some differences at 3100, 1483, 1223, 941, and 680 (amide V) cm−1 were also observed, which is consistent with our previous result in PA1012 homopolymer.48 Different from the PA1012 homopolymer, the amide II band (stretching band of C–N group and deformation vibration band of N–H group) at 1560 cm−1 shifts to lower wavenumber with the crosslinking network. For the film with crosslinking network (blue curve in Fig. 3), the absorbances of the bands at 2918 and 2859 cm−1 (corresponding to the –CH2– asymmetrical and symmetrical stretching modes, respectively) are saturated due to the thicker film-thickness, lead to the not sharp bands.
It was reported that the secondary amine groups (R1–NH–R2) are obtained by the reaction between the end groups of various polyamide homopolymers (known as diamine coupling) and act as branch points that react with carboxyl groups to form the branched structures (ESI, Scheme S1†).49,50 For polyamide melts at high temperature, the depolymerization of polyamide chains can also occur due to the ammonolysis reactions, and the depolymerized segments with –CONH2 end-group can further reacts with the secondary amine group to form the crosslinking structures.61 In other words, the chain scission and crosslinking are probably occurring simultaneously. As a result, both the end groups and the amide group in the repeating unit of polyamide chains can act as a crosslinking point. Based on the above reaction mechanisms, we propose a possible mechanism for the crosslinking reaction of PEBA elastomers as Scheme 1. The crosslinking structure established by the amide group of the repeating units of PA12 HS provides an additional elasticity for the studied PEBA elastomers. Although the proposed mechanism seems reasonable according to the literature, more research is needed to clarify it.
It should be noted that the scaling law of G′ ∼ ω2.0 is invalidated at 200 °C (Fig. 2a), and the crosslinking reaction can be ignored at Tiso < 210 °C due to the extremely low reaction rate. According to the AFM images of Pebax® 5533 films quenched from the melt state,41 the microphase separation can be developed within several minutes. Therefore, it's reasonable the P35D melt is not homogenous one-phase but microphase separated at 200 °C, the physical microphase structure is strong enough to lead to the deviation of G′ ∼ ω.2.0 However, the microstructure of P35D melt may be dominated by the crosslinking network after the isothermal at 200 °C for an infinite period of time, even if the reaction rate is extremely low.
In block copolymers, it's well known that the microphase separation can lead to the deviation of the scaling law of G′ ∼ G′′2.0 in the Han-plot, for which is an important means of detecting the existence of microphase separation.2,21–23,27,60 Fig. 4 shows the Han-plots obtained from P25D, P35D, and P40D after the 2 hours isothermal treatment at various Tiso. It was found that the homogenous criterion of G′ ∼ G′′2.0 is only valid in 170–190 °C for P35D. Considering that no G′ plateau is detected in the Han-plots and the magnitude of G′ does not increase significantly with time (see ESI, Fig. S1†), P25D and P40D are considered to be microphase separated at 170 °C. At higher temperatures, such as 210–270 °C, the evident crosslinking network has been developed in all the three copolymers.
Fig. 4 Han-plots obtained from (a) P25D, (b) P35D, and (c) P40D melts after the 2 hours isothermal treatment at various Tiso from 170–270 °C. |
Due to the existence of crosslinking network, the microphase separation transition temperature cannot be determined precisely by the typical means such as Han-plot and SAXS in our system. The microphase separation originates from the interaction between the hard- and soft-segments, the apparent homogenous morphology and the possible short-range order structures may exist in the disorder state of block copolymers.2 In contrast, the long-range order structures are formed in the microphase separation region in the phase diagram. In other words, the microphase separation temperature can be defined by the relative phase domain size. Herein, the transition temperature can be analog with the binodal temperature in polymer blends, which are induced by the concentration fluctuation. Therefore, the Tb of P35D can be estimated by using the Ajji and Chopin's method.54–56 Fig. 5a shows the temperature dependence of G′ and G′′ with various heating rates. These curves merged well at low temperatures, where the homogenous criterion was satisfied. For each heating traces, the Tb was determined by the G′ minimum62 at where G′ starts to upturn because of the strong enough concentration fluctuation. Meanwhile, Ts was determined from the extrapolation of the linear regression on the x-axis of the plots of (G′′2/G′T)2/3 versus the reciprocal of temperature 1/T at a given heating-rate (shown in ESI, Fig. S4†) according to the Ajji and Chopin's model. Noted that Ts was obtained with the experimental data at the apparent homogenous region (T < Tb) for ensuring accuracy. Because the microphase separation is a time-dependent behavior, the heating rate effect on Tb and Ts should be considered. The good linear relationships for Tb and Ts versus heating rate are seen in Fig. 5b, the transition temperatures at equilibrium state (denoted as and ) are determined to be 200.8 and 229.6 °C, respectively, by the extrapolation on the y-axis. Therefore, the PEBA copolymers exhibit an LCST-type phase diagram, also called lower disorder-to-order transition behavior. It's reminded that the applied SAOS may shift the binodal line in the phase diagram and induce a temperature-gap with the quiescent state, the gap is smaller at the lower frequency63,64 In this work, all the temperature sweep measurements were performed at ω of 0.5 rad s−1, thus, the is considered to be reliable and close to that obtained at the quiescent state. The obtained is consistent with the Han-plot result that the microphase separation has been developed in P35D at 200 °C. Although the phase diagram of Pebax® elastomers has not yet been constructed, it may be a dynamic asymmetric system because there is a considerable gap between Tc,HS and Tc,SS (Table 1).
In our system, once the microphase separation behavior occurs, it's difficult to distinguish the contribution from the crosslinking process on dynamic moduli in the initial stage. Moreover, the crosslinking reaction is time-dependent with a slow rate at lower temperatures. Therefore, the Tcross can only be approximately determined by the following experiments: one fresh sample was firstly heated to 170 °C and hold isothermal for 2 hours, then quickly jumped to various Tiso and hold isothermal for 2 hours, finally cooled back to 170 °C to check whether the microstructure is recoverable. Fig. 6a clearly shows that G′ of P35D returned to the equilibrium value (black color) at 170 °C within 2 hours in the case of Tiso = 190 °C (below ), the reversible behavior indicates it's the homogenous state at 190 °C which is in good agreement with the Han-plots (Fig. 4b). In , the rheological properties are only affected by the chain entanglement. Although G′ shows an upward trend at Tiso of 203 °C within two hours, G′ eventually returns to the equilibrium value at 170 °C, implying that the content of crosslinking structure is so low that can be ignored within the measurement at 203 °C. On the other hand, the G′ cooled down from Tiso of 207 °C remained almost constants and three-folds higher than the equilibrium value at 170 °C, the considerable gap indicates the microstructure has been dominant by the crosslinking structure. It can be concluded that the exactly Tcross is in the range of 203–207 °C. When Tiso > Tcross, the crosslinking network develops rapidly, and becomes the dominant factor in determining the microstructure and rheological properties within 2 hours.
Based on our experiments, the microstructure evolution of PEBAs upon heating can be divided into ternary domains: the apparent homogenous, microphase separation, and crosslinking domains, by the determined and Tcross. Taking P35D as an example, it is apparent homogenous state (the hard and soft segments are compatible at the molecular level) in the temperature range of . Meanwhile, the rheological properties and microstructure are reversible and depending on the temperature. In the range of , a microphase-separated P35D melt was obtained after the 2 hours isothermal treatment, the content of crosslinking network is negligible due to the slow reaction rate. The microstructure is mainly affected by the microphase separation, the equilibrium state at a given temperature can be reached over a period of time. However, once the P35D melt was heated to temperatures above Tcross, the content of crosslinking network (or crosslinking density) increases significantly, and becomes the dominant factor within a short period, resulting in the irreversible evolutions in microstructure. As presented, the rheological properties are also irreversible with temperature/time in this temperature domain. Not only PEBA copolymers, the reported irreversible effect of thermal history on the rheological properties in the TPU elastomers20 can also be explained by this mechanism.
Unfortunately, no intensity peak associated with the long-range ordered structures was detected after the isothermal-treatment in the P35D's SAXS experiments (ESI, Fig. S5†), even in the microphase-separated domains. It may be caused by the poor contrast of electron cloud density between the hard- and soft-segments. In addition, no evident long-range order structure was observed by the TEM technique for the ultra-thin sections of P35D films quenched from Tiso above after the 2 hours isothermal process (shown in ESI, Fig. S6†). There are two possibilities: the first one is that interdomain distance is too short to be clearly seen. The interdomain distance can be as small as serval nanometers to tens of nanometers for Pebax® elastomers,41,65 which is much smaller than the specimen thickness (ca. 90 nm). Therefore, the obtained ultra-thin section may contain tens of layers of this kind of structure, resulting in the homogenous-like morphology. The other possibility is that the crosslinking network inhibits the formation of the long-range order structures at T > Tcross. For the three studied elastomers, the lower ΔHm,HS and ΔHc,HS of the treated films after the isothermal process at 270 °C compared to the respective raw pellets implies that the presence of crosslinking network can reduce the regularity of the polymer chains. In other words, the chain mobility of P35D melt is probably restricted by the formed crosslinking structures.
Tiso (°C) | Young's modulus (MPa) | Strain-at-break (%) | Stress-at-break (MPa) |
---|---|---|---|
Untreated | 5.0 ± 0.3 | 228.7 ± 1.4 | 222.8 ± 41.7 |
180 | 4.8 ± 0.1 | 219.0 ± 11.5 | 270.6 ± 48.5 |
230 | 5.5 ± 0.2 | 235.1 ± 0.7 | 316.4 ± 57.7 |
270 | 5.2 ± 0.2 | 211.7 ± 10.4 | 326.3 ± 60.2 |
Determination of the critical temperatures (Tm,HS, and Tcross) is important for regulating the microstructure and studying PEBA's structure–property relationship. It should be noted that these critical temperatures are depending on the content of PA component. For example, the Tm,HS of PEBA increases with the content of polyamide segment, it is possible to even higher than . In that case, no homogenous state can be obtained. Although this work has demonstrated the competition between the microphase separation and crosslinking structures for the microstructure of PEBA, the mechanism is still not well understood from the molecular level viewpoint.
These results provide a deep understanding of the microstructure evolution of the PEBA elastomers. Further investigations of the effects of microstructure evolution and chemical crosslinking on the crystallization behaviors of PEBA are undertaking in our lab, and the results will be reported in the near future.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra10627e |
This journal is © The Royal Society of Chemistry 2021 |