Nanostructure and Linear Rheological Response ... - ACS Publications

Oct 8, 2015 - State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,...
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Nanostructure and Linear Rheological Response of Comb-like Copolymer PSVS‑g‑PE Melts: Influences of Branching Densities and Branching Chain Length Yichao Lin,†,‡ Yanhui Wang,† Jun Zheng,†,‡ Kun Yao,†,‡ Haiying Tan,†,‡ Yaotao Wang,†,‡ Tao Tang,*,† and Donghua Xu*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡ Graduate School of the Chinese Academy of Sciences, Beijing 100039, China S Supporting Information *

ABSTRACT: Comb-like poly(styrene-co-4-(vinylphenyl)-1-butene)-g-polyethylene copolymers (PSVS-g-PE) with various branching parameters were synthesized to study the influence of branch chains on morphology (at melt state) and linear rheological response of the copolymers. The results showed that both the branching density and branch chain length of PSVS-g-PE copolymers strongly affected linear rheological behavior of the copolymers, resulting from the formation of different microphase separation structure in the melt state. PSVS-g-PE copolymers with low branching density (2.3−3.5 branch chains per 100 repeating units of the backbone) showed a microphase-separated structure at the melt state, and a typical rheological characteristic for network-like structure was observed. Furthermore, the type of microphase-separated structure at the melt state strongly influences the applicability of the time−temperature superposition (TTS) principle. As a result, the TTS failure was observed in the modulus curves for PSVS52.7-3.5-PE4.9 (poor-order lamellar structure) and PSVS54.4-2.7-PE10.7 (long tubular structure). In contrast, the PSVS-g-PE sample with high branching density (16.6−24.5 branch chains per 100 repeating units of the backbone) showed homogeneous phase structure and normal rheological behavior, similar to linear or comb-like homopolymers. The gel-like state appeared in a limited frequency regime (a plateau regime of tan δ versus ω) during decreasing the frequency from the high frequency regime in these comb-like copolymers. relaxes at first after the glassy regions, and then the backbone relaxes.7 Comparatively, the rheological behavior of topological copolymers with branched architecture has been studied less extensively, such as comb-like graft copolymers, in which the composition of the branch chain is different from that of the backbone.13,16−18 At the same time, the dynamics of a comblike graft copolymer is much more complex than that of a comb-like graft homopolymer. On one hand, the chain architecture (both branch density and branch length) influences the topology; on the other hand, the topology results in the formation of various microphase-separated structures. From the fundamental scientific view, it is a very interesting issue about the interplay of chain topology and microphase-separated morphology on melt rheological properties of these nonlinear copolymer systems. As we know, the chain architecture of copolymers may lead to the formation of different microphase structure in the bulk, which affects the properties of polymers, such as rheological properties. For example, the occurrence of a phase-separated

1. INTRODUCTION It is a hot topic how polymer architecture affects the microstructures and properties of polymer materials.1−6 For example, the presence of a small amount of long chain branching (LCB) has a dramatic effect on linear rheological behavior in well-entangled polymers comparing with the linear counterpart with the same molecular weight, especially in the low shear frequency range.7 Particularly the branching density and molecular weight of the branch chains affect the movement of the backbone and rheological responses of these complex macromolecules. The relationship between structural parameters and rheological properties of comb-like polymers is very important to understand the influence of branch chains on melt behavior of polymers. However, most of the previous studies are focused on LCB homopolymers (the same composition of the branch chain as that of the backbone),7−12 especially LCB homopolymers with a low branching degree (fewer than 100 branches per 10000 carbon atom of the backbone).7−10,12−15 These comb-like polymers are homogeneous systems due to the same composition of both the backbone and the side chain; thus, the time−temperature superposition (TTS) is obeyed. Generally, these LCB homopolymers usually show hierarchical relaxations from the linear rheological result; that is, the branch © XXXX American Chemical Society

Received: June 19, 2015 Revised: August 31, 2015

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DOI: 10.1021/acs.macromol.5b01335 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules microstructure strongly affects the linear viscoelastic response of block copolymer systems.4,19 In some cases, phase-separated microstructure results in the failure of the time−temperature superposition (TTS) at a certain frequency region.20,21 Recently, Dealy et al.22 and Yu et al.23 have found a similar phenomenon in olefin multiblock copolymers, in which a lamellar mesophase is present in the melt at temperatures well above the melting point. In another report, multiblock vinyl acetate−vinyl alcohol copolymers exhibited the failure of TTS due to the formation of crystalline or amorphous microstructures, while the random copolymers with the same composition obeyed the TTS.24 Compared to block copolymers, there are more molecular parameters influencing the morphology and physical properties of graft copolymers besides volume fraction and degree of miscibility, such as branching density and branch chain length. In the limited reports about the rheological response of graft copolymers, the results showed that the hierarchical relaxations were also observed in the dynamic moduli−frequency master curves.16,17 The TTS could be successfully applied in the poly(acrylic acid-g-styrene) (PAA-g-PS) graft copolymers (with lamellar morphology)16 and poly(tert-butyl acrylate-g-styrene) (PtBA-g-PS) graft copolymers (with disordered microdomains).17 In the former case, the authors thought that the applicability of TTS for the PAA-g-PS graft copolymers resulted from that the relaxation time scales of the PAA and PS domains were very similar due to small difference in both Tgs. However, it is still not very clear which kinds of microphase-separated structure result in the applicability or failure of TTS in the copolymer systems. Recently, the linear rheological property of poly(macromonomer)s (or bottlebrush) with dense branches has attracted academic research interests.25−32 Some reports have shown that the poly(macromonomer)s remain unentangled due to too high branching density (2000−5000 branches per 10 000 backbone carbon atom, depending on synthetic methods); even molecular weights of the backbone and branch chain are higher than the corresponding critical entanglement molecular weights.30−32 For poly(macromonomer)s, in which the composition of branch chain is different from that of the backbone, microphase separation structures cannot be formed due to low volume fraction of the backbone originating from high branching density.30,31 Here we wonder what will happen for the comb-like copolymers with branching degree in the range of more than 100 branches per 10 000 backbone carbon atom but lower than that of poly(macromonomer)s. For this kind of comb-like copolymer, the effects of the branch structure on the rheological properties are still not well-understood due to the intricacy of this issue. In this work, a series of comb-like PSVS-g-PE copolymers with high branching degree (from more than 100 branches to 1225 branches per 10 000 backbone carbon atom, which is equal to from more than 2 branches to 24.5 branches per 100 backbone repeat units) and various branch length were successfully synthesized using our reported method shown as Scheme 1.33−35 Using these comb-like copolymers, we study how the presence of branch chains with high branching density (different from bottlebrush polymers) affects microphaseseparated morphology and the rheological properties of comb-like copolymers. Furthermore, our study is focused on the influences of the phase-separated microstructures (lamellar, cylindrical, or spherical morphology) on linear viscoelastic response of comb-like copolymer systems. We wonder how the

Scheme 1. Synthesis Route for the Comb-like PSVS-g-PE Copolymers

microstructure morphology of comb-like PSVS-g-PE influences linear rheological response of the copolymers.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Comb-like PSVS-g-PE Graft Copolymers. The synthesis route of comb-like PSVS-g-PE copolymers with different branch structural parameters is shown in Scheme 1. The detailed information about the synthesis and characterization of the backbone precursor poly(styrene-co-4-(vinylpheneyl)-1-butene (VSt)) (PSVS) and iodinated PSVS copolymers (as the precursor) could be found in our previous work.34 1,4-Polybutadiene (1,4PB, as branch chain) was synthesized by living anionic polymerization of butadiene. The detailed information about the synthesis and characterization of 1,4PB, PSVS-g-1,4PB, and the final product PSVS-g-PE could be found in the Supporting Information. The results about the PSVS-g-PE are shown in Table 1. The synthesized comb-like copolymers were described by the following nomenclature PSVSx-y-PEz, where x is the number-averaged molecular weight of the backbone PSVS (Mn,bb, kg/ mol), y is the average number of side chains per 100 repeating units of the backbone, z is the calculated number-averaged molecular weight of side chain (PE) on the base of the corresponding 1,4PB. The copolymerization behavior of the two monomers St and VSt is also studied in the Supporting Information. The result shows that the copolymerization of the two monomers St and VSt tended to general ideal copolymerization (r1r2 = 1.07), implying that the branch chains linked to the backbone via VSt were randomly distributed in the resultant PSVS-g-PE copolymers. 2.2. Characterization. Rheological experiments were performed on an ARES-G2 rheometer (TA Instruments) with 25 mm parallel plate geometry under nitrogen purge. Loading conditions of the samples in the rheometer are as follows: the sample was put onto the 25 mm parallel plate at experimental temperature for about 10 min to melting, and then the sample was slowly pressed until the gap was about 0.75 mm using the axial stress below 0.5 N. At last, the samples were kept about 10 min before testing in order to eliminate the axial stress (below 0.001 N). Strain sweep experiments were performed first to determine the linear strain regime. Oscillatory frequency sweep was performed from 0.05 to 500 rad/s with a strain in the linear strain regime. Linear frequency sweep were performed at different temperatures ranged from 120 to 270 °C. Time−temperature superposition was performed for the frequency sweep results of the comb-like copolymers, and the reference temperature was 150 °C. The height and phase images of the comb-like copolymers at 150 °C were acquired using tapping-mode atomic force microscopy (AFM) with a hot stage (Nanoscope IIIa MultiMode, Digital Instruments). The sample for AFM observation was prepared by spin-coating on clean silicon wafer at high temperature. The PSVS-gPE solution was about 5 mg/mL in o-dichlorobenzene. The silicon wafer with sample was dried under vacuum at 80 °C overnight. Before observation, the specimen was kept for 40 min at 150 °C in order to obtain stable morphology. Small-angle X-ray scattering (SAXS) measurements were performed at beamline 1W2A, BSRF, Beijing, China. The distance from sample to detector was 3015 mm, the wavelength of X-ray was 0.154 nm, and B

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Macromolecules Table 1. Molecular Characteristics of Graft Copolymers PSVS-g-PE sample e

PSVS65.8-24.5-PE4.9 PSVS52.7-3.5-PE4.9e PSVS45.3-22.8-PE10.7e PSVS54.4-2.7-PE10.7e PSVS65.8-20.0-PE19.4e PSVS52.7-2.3-PE19.4e PSVS65.8-16.6-PE36.7e PSVS52.7-2.6-PE36.7e

Mwa (kg/mol)

PDIa

φbrb

sbbc

sbrc

qd

nf

936.4 166.3 1123 183.1 3076 286.3 4986 763.4

1.03 1.05 1.04 1.05 1.05 1.03 1.03 1.06

0.929 0.654 0.964 0.760 0.977 0.830 0.985 0.913

3.8 3.0 2.6 3.1 3.8 3.0 3.8 3.0

2.7 2.7 5.9 5.9 10.8 10.8 20.4 20.4

8.2 57.1 8.7 74.1 10.0 87.0 12.0 76.9

133.4 17.3 85.5 13.8 108.9 11.4 90.4 12.9

By GPC-TALLS in TCB at 150 °C. bVolume fraction of the branches (ρ(PS) = 1.05 mg/mL and ρ(PE) = 0.90 mg/mL). cThe dimensionless number of entanglements which is defined as sbb for the number of backbone entanglements and sbr for the number of branch entanglements (s = Mw/Me); Me(PS) = 17.3 kg/mol, Me(PE) = 1.8 kg/mol). dThe average number of the C atom between two adjacent branches. ePSVSx−y-PEz, where x is the Mn of the backbone PSVS, y is the average number of branch chains per 100 repeating units of the backbone, and z is the calculated molecular weight based on the Mn of the precursor 1,4PB. fThe side chain number per a comb-like copolymer chain. a

then the effective range of the scattering vector q (q = (4π sin θ)/λ, where 2θ is the scattering angle and λ is the wavelength) was 0.1−1.0 nm−1. In order to obtain stable morphology, the specimen was kept for 40 min at each testing temperature (≥150 °C) before collecting data. Each pattern was collected within 120 s. All the two-dimensional (2D) SAXS patterns were then background corrected and normalized using the standard procedure. The scattering patterns after calibration were averaged over all directions at a constant q, resulting in onedimensional (1D) scattering intensity curves.

profiles of scattering intensity as a function of the scattering vector (q), indicating the presence of microphase separation structure in these melts. It was found that the primary peaks kept the almost same position at different temperatures (PSVS52.7-2.3-PE19.4 as an example in Figure 2c, from 150

3. RESULTS AND DISCUSSION 3.1. Morphology of Comb-like PSVS-g-PEs Copolymer Melts. As we know, the chain architecture is a very important factor in determining morphological behavior of copolymers. In order to explore the effects of branching density and branch length on the morphological behaviors of PSVS-g-PE, all the comb-like copolymers were characterized by SAXS. Here the morphology characterization is focused on the melt state to provide a basis for studying the influence of microstructure on rheological response of the copolymers. Figure 1 shows the corresponding SAXS profiles for the PSVS-g-PE with low branching density (2.3−3.5 branch chains per 100 repeating units of the backbone) in the melt state at different temperatures. There was a broad primary peak in the SAXS Figure 2. 1D SAXS intensity distribution for (a) PSVS65.8-24.5PE4.9, (b) PSVS45.3-22.8-PE10.7, (c) PSVS65.8-20.0-PE19.4, and (d) PSVS65.8-16.6-PE36.7.

to 230 °C), where the temperatures were the same as those used for the rheological measurements (see the following section). In contrast, there were not obvious peaks in the SAXS profile of scattering intensity for all the comb-like copolymers with high branching density (16.6−24.5 branch chains per 100 repeating units of the backbone) (Figure 2), implying that there was no the formation of microphase separated structures in these melts. In order to prove the above results, the AFM observation at 150 °C was carried out. Figure 3 shows the AFM height images of the comb-like copolymer melts with low branching density at 150 °C. In the cases with low branching density, for PSVS52.73.5-PE4.9, it is difficult to obtain clear AFM image at 150 °C, so the result is not shown here. All other three comb-like copolymers with low branching density presented microphaseseparated structure (nanostructure) (Figure 3). In contrast, there was no microphase-separated structure in the comb-like copolymers with high branching density (Figure 4). This

Figure 1. 1D SAXS intensity distribution profiles for (a) PSVS52.73.5-PE4.9, (b) PSVS54.4-2.7-PE10.7, (c) PSVS52.7-2.3-PE19.4, and (d) PSVS52.7-2.6-PE36.7. C

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lations, in which order microphase-separated structure (from lamellar phase structure to sphere phase structure) is observed, as the volume fraction of the backbone component is changed from 50% to 17%.36 In this work, for the comb-like copolymers with low branching density, the volume fraction (ϕbb) of the backbone component is in the region of 9−35% (Table 1). Generally, the morphologies with specific ordered phase structure can be identified by SAXS measurements based on the spacing and intensity ratios among the measured Bragg peaks for long-range order at different angles. In contrast, the disordered state only shows short-range correlations but no long-range order, meaning that there is only a broad primary peak at the scattering vector q* accompanied by the disappearance of higher order peaks in the SAXS profiles for the disordered state of a block copolymer.37,38 For PSVS52.73.5-PE4.9, the SAXS profile exhibits a broad primary peak at the scattering vector q* along with a weak peak at 2q* (Figure 1a). This is indicative of a poor-order lamellar morphology,38 as expected due to the volume fraction of backbone (ϕbb = 34.6%). The system with scattering peaks at q*, √3q*, 2q*, and √7q* can be ascribed ordered 2D hexagonal arrangement of cylindrical domains.4,39 The SAXS profile of PSVS54.4-2.7PE10.7 shows a broad primary peak with very weak higher order peaks at √3q*, 2q*, and √7q* (Figure 1b), indicating poor-order hexagonal packs (HEX) of cylindrical microdomains. Meanwhile, the SAXS profile of PSVS52.7-2.3PE19.4 shows a broad primary peak with weak higher-order peaks at √3q* and √7q* (Figure 1c), indicating that PSVS52.7-2.3-PE19.4 has also poor-order HEX cylindrical microdomains. Furthermore, combining with the AFM images at 150 °C (Figure 3), the PSVS54.4-2.7-PE10.7 (ϕbb = 24%) closely shows long tubular or bicontinuous morphology, and PSVS52.7-2.3-PE19.4 shows cylindrical morphology. The order body-centered cubic (BCC) sphere phase has been reported with peaks at q*, √3q*, and √6q*.39 For PSVS52.7-2.6PE36.7, the SAXS profile only exhibits a broad primary peak at q* (Figure 1d) in this work. According to the AFM image of PSVS52.7-2.6-PE36.7 in Figure 3d and the volume fraction of backbone (ϕbb = 8.7%), PSVS52.7-2.6-PE36.7 may shows disorganized sphere morphology. 3.2. Linear Rheological Behavior of Comb-like Copolymers PSVS-g-PEs Melt. The above results show that the branch structure of comb-like PSVS-g-PEs strongly affects the microstructure morphology of copolymers in the melt state, especially the branching density. The linear rheological properties of the comb-like PSVS-g-PE were studied by frequency sweep in linear strain regime. Figure 5 presents the master curves of the storage modulus (G′) and loss modulus (G″) for the comb-like PSVS-g-PE copolymers with high branching density at the reference temperature of 150 °C. As shown in Figure 5, all these comb-like copolymers show a typical rheological behavior in which the G′ is smaller than the G″ in the low frequency regions. Generally, it is expected that comb-like polymers relax sequentially. The chain segments (glassy mode) should relax first, followed by the branch chains, and the whole polymer should be the last to move.3,40 However, from Figure 5, it could be seen that the branch chain length remarkably affected the relaxation behavior of the copolymers. When the branch chain is PE4.9 and PE10.7, there is only a weak transition region in the entire measured frequency region, coming from the whole copolymer relaxation of these two samples (Figures 5a and 5b), which is similar to polymer melts with low molecular weight (Mn < Me). Owing to

Figure 3. AFM height images at 150 °C for (a) PSVS54.4-2.7-PE10.7, (b) PSVS52.7-2.3-PE19.4, and (c) PSVS52.7-2.6-PE36.7.

Figure 4. AFM height images at 150 °C for (a) PSVS65.8-24.5-PE4.9, (b) PSVS45.3-22.8-PE10.7, (c) PSVS65.8-20.0-PE19.4, and (d) PSVS65.8-16.6-PE36.7.

confirms the strong influence of branching density on the melt morphology of comb-like copolymers. A possible reason for the formation of homogeneous phase structure in the comb-like copolymers with high branching density is that high branching density of branch chain prevents the formation of the backbone phase due to the difficulty of the backbone contacting each other or low volume fraction of the backbone component. This issue will be discussed in a later section. As we know, the volume fraction of branch chains in the comb-like copolymer depends on both the branch density and branch length; for example, the volume fraction of PE branch chain (ϕbr) is 0.830 and 0.977 for PSVS52.7-2.3-PE19.4 and PSVS65.8-20.0-PE19.4, respectively (Table 1). Liu et al. have studied the microphase separation structure of comb-like copolymers in melts by dissipative particle dynamics simuD

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Figure 5. Master curves of storage moduli (G′), loss moduli (G″), and tan δ versus angular frequency (ω) for the comb-like copolymers: (a) PSVS65.8-24.5-PE4.9, (b) PSVS45.3-22.8-PE10.7, (c) PSVS65.8-20.0-PE19.4 and (d) PSVS65.8-16.6-PE36.7 at the reference temperature of 150 °C.

observe the terminal relaxation of the whole copolymer in the experimental measured frequency region. Figure 6 shows the plots of complex viscosity (|η*|) versus angular frequency (ω) of these comb-like copolymers. When

high branching density of these comb-like copolymers, it is difficult for the backbone from different comb-like copolymers to contact each other; as a result, no entanglement between the backbones was formed. Another important reason is that the excluded volume effect makes the side chain to become more stretch to some extent which could greatly decreases the entanglement of side chains. In all, there are few entanglements for PSVS65.8-24.5-PE4.9 and PSVS45.3-22.8-PE10.7. So the two copolymers present a viscouslike behavior (G″ > G′). For pure viscous liquid, the phase angle δ is 90°, so here we can say viscous dominant as δ is larger than 45° (e.g., G″ > G′). With increasing the length of PE branch chain to 19.4 kg/mol, the appearance of the plateau region (G′ > G″) in the intermediate frequency region indicates the elastic nature. For pure elastic solid, the phase angle δ is 0°, so here we can say elasticity dominant as δ is smaller than 45° at higher frequency (e.g., G′ > G″) in the PSVS65.8-20.0-PE19.4 melts. There are two transition regions from the curves of tan δ versus ω for the PSVS65.8-20.0-PE19.4 (Figure 5c), that is, the minimum value at the higher frequency region and the shoulder transition in the lower regions of measured frequency, meaning two relaxation processes.14 The two-step relaxation corresponds to the relaxation of the branch in the middle frequency region and the slow motion of the backbone in the low frequency region, respectively.3,12,14 Considering high branching density of branch chain and the low effective concentration of the backbone in these PSVS-g-PE samples, it is difficult for the backbones from different comb-like copolymers to contact each other, so no entanglement between the backbones was formed. Here we think that the shoulder transition for tan δ versus ω in lower frequency region may result from the relaxation of the whole comb-like copolymer.28,30 When the branch chain is increased to 36.7 kg/mol, there is only one transition in the measured frequency region (Figure 5d), which should be ascribed to the relaxation of PE branch chains. It is believed that the serious entanglement of branch chain (sbr = 20.4 for PE36.7) leads to the slow relaxation for the branch chains and also for the whole comb-like copolymer, so it is difficult to

Figure 6. Complex viscosity (|η*|) versus ω for the comb-like copolymers with high branching density.

the side chain became long, the |η*| increased dramatically at low frequency (from 76 Pa·s for PSVS65.8-24.5-PE4.9 to 5.15 MPa·s for PSVS65.8-16.6-PE36.7). This results from the slower relaxation of comb-like polymers with longer branch chain. The |η*| of the two comb-like copolymers with short PE branch chain gradually level off in lower frequency regime, but those of the two comb-like copolymers with long PE branch chain gradually increased because the whole comb polymers did not relax fully in the measured frequency regime. Figure 7 shows the results of linear frequency sweep for these comb-like copolymers with low branching density at the reference temperature of 150 °C. Interestingly, the G′ is higher than G″ in the whole experiment frequency regime for all the samples, and the G′ shows weak dependence of angular frequency (ω) at the lower frequencies. Because the backbone PS is incompatible with PE branch chain at the melt state, the appearance of quasi-plateau in the plot of G′ with angular frequency at low frequency should be caused by the microphase-separated structure in the graft copolymer melt, showing a network-like behavior.19 A similar rheological E

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Figure 7. Master curves of storage moduli (G′), loss moduli (G″), and tan δ versus angular frequency (ω) for the comb-like copolymers: (a) PSVS52.7-3.5-PE4.9, (b) PSVS54.4-2.7-PE10.7, (c) PSVS52.7-2.30-PE19.4, and (d) PSVS52.7-2.6-PE36.7 at the reference temperature of 150 °C.

When the side chain is the same, the rheological behavior of comb-like copolymers with different branching density was further compared below. As shown in Figure 9, the G′, G″, and

phenomenon has been observed in PTBA-g-PS and PAA-g-PS graft copolymers.16,17 The crossover point of G′ and G″ at low frequencies cannot be detected in the master curves of these graft copolymers, resulting from the microphase-separated structure of the samples. Another interesting phenomenon is that the failure of time− temperature superposition (TTS) principle is observed in the modulus curves for PSVS52.7-3.5-PE4.9 and PSVS54.4-2.7PE10.7 (Figures 7a and 7b). Vertical and horizontal shifting of the data to single curves using identical shift factors for G′ and G″ was not possible for these two samples with short PE arms. When the PE chain was increased to 19.4 and 36.7 kg/mol, the TTS principle could be applied in the modulus curves of PSVS52.7-2.3-PE19.4 and PSVS52.7-2.6-PE36.7. Furthermore, both these two samples showed a minimum value of tan δ at the frequency of ∼103 rad/s, which is believed to result from the branch chain relaxation (Figures 7c and 7d).12 Interestingly, as shown in Figure 8, the |η*| of all the comb-like copolymers showed a upward trend with the decreasing frequency in the low frequency region, independent of the length of PE branch chains. In summary, the melt rheological behaviors are quite different between the comb-like copolymers with high branching density and those with low branching density. More detailed discussion can be seen in the following section.

Figure 9. (a) Master curves of storage moduli (G′), loss moduli (G″) versus ω and (b) the complex viscosity (|η*|) versus ω for the comblike copolymers PSVS52.7-2.3-PE19.4 and PSVS65.8-20.0-PE19.4.

|η*| of PSVS52.7-2.3-PE19.4 and PSVS65.8-20.0-PE19.4 are almost the same in higher frequency region. This may be the reason that the comb-like copolymers mainly show side chain relaxation in this frequency region. In the case of PSVS65.820.0-PE19.4, the G″ is larger than G′ (Figure 9a) in lower frequency region; meanwhile, the η* gradually levels off (Figure 9b). In contrast, for PSVS52.7-2.3-PE19.4, the G′ is larger than G″, and the η* increases continuously with the frequency decreasing in lower frequency region. The above results mean

Figure 8. Complex viscosity (|η*|) versus ω for the comb-like copolymers with low graft density. F

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tubular structure) (Figures 7a and 7b). The previous report has shown that the TTS failure is found in some cases, such as immiscible polymer blends,45 filled,46 and nonfilled47 styrene− butadiene rubber and arborescent polystyrene-g-isoprene copolymers.44 As shown in Figures 1 and 3, the PSVS-g-PE copolymers with low branching density showed poor-order lamellar, long tubular, poor-order cylindrical, and disorganized spherical morphology structure due to various PE length (or volume composition). Apparently, the morphology of microphaseseparated structure appears to be an important factor affecting TTS fitting of the rheological data. This shows that whether each component in the copolymers retains their own relaxation properties (due to different temperature-dependent monomeric friction coefficients) depends on the type of microphase separation structure. Recently, Handge et al. have reported a similar phenomenon; that is, the TTS principle was not well obeyed in the modulus curves for PS-b-PMMA diblock copolymers with a lamellar morphology, but the samples with cylindrical morphology obeyed the TTS principle.4 As we know, both Tgs of PS and PMMA are very close in PS-bPMMA diblock copolymers,4 and the Tg of PS is much higher than that of PE in this work. So it is difficult to explain that the applicability of TTS for the copolymers with microphaseseparated structure results from that the relaxation time scales of two phases are very similar due to small difference in both Tgs.16 We think that the volume fraction of the backbone phase for the PSVS52.7-3.5-PE4.9 and PSVS54.4-2.7-PE10.7 is comparatively high (24% and 35%), which more easily result in more than one relaxation properties, leading to the failure of the TTS principle. To further distinguish the influence of branching parameters and volume fraction of PE branching chain (ϕbr) on the microphase morphology and rheological behavior of PSVS-gPE, two PSVS-g-PE samples with similar ϕbr (0.913 to 0.929) but apparently different branch length and branching densities (Table 1) are compared. PSVS52.7-2.6-PE36.7 (low branching density and long branch chain) showed microphase-separated spherical structure (Figures 1d and 3c). In this case, the solidlike rheological behavior was observed. In contrast, PSVS65.824.5-PE4.9 (high branching density but short branch chain) showed a typical rheological behavior of unentangled polymers melts in the measured frequency regime (Figure 5a) and homogeneous phase structure (Figures 2a and 4a). These results demonstrate that the branching parameters (especially branching density) play a key role in the morphology and rheological behavior of comb-like PSVS-g-PE copolymers. It is very interesting why the branching density shows strong effect on the morphology and rheological behavior of PSVS-gPEs at melt state. For PSVS-g-PEs with the branching density 2.3−3.5 branch chains per 100 repeating units of the backbone, the average number of C atom between two adjacent branches (q) is about 57−87 (Table 1). It is expected that the PS segments between two adjacent PE branches could aggregate at melt state because of the immiscibility between PS and PE. The PS chain segments aggregate to form one nanostructured phase, and PE side chains are distributed around the PS phase to form the continuous phase. The microphase separated structure such as sphere-like structure can be considered as a cross-linking point for the surrounding PE branch chains, while the PE side chains around PS phase can entangle or not entangle together (depending on the branch length), which results in the formation of a physical network structure. This

that PSVS65.8-20.0-PE19.4 with a high branching density could relax in low frequency region, while PSVS52.7-2.3-PE19.4 with a low branching density could hardly relax, due to the formation of microphase-separated structures. The elasticity dominant rheological behavior of the comb-like copolymers with low branching density results from the formation of microphase-separated network morphology (such as PSVS52.72.3-PE19.4), while the formation of homogeneous phase structure in the comb-like copolymers with high branching density (such as PSVS65.8-20.0-PE19.4) results in normal linear rheological behavior, similar to that of linear or comb-like homopolymer melts. 3.3. Discussion. The above results demonstrate that the branching density plays a key role in the morphology and rheological behavior of comb-like PSVS-g-PE copolymers. Very interestingly, in the cases of PSVS-g-PE copolymers with high branching density, it can be found from the curves of tan δ versus ω (Figure 5) that there is always a frequency region in which the curves of G′(ω) and G″(ω) are nearly parallel to each other (a plateau regime of tan δ versus ω) before the sample becomes more viscous with the decrease of ω. GarciaFranco et al. have found that there is similarity between gelation and viscoelastic behavior of LCB polymers (or comblike polymers);41 that is, the storage and loss moduli (G′ and G″) of a critical gel obey a scaling law with the same exponent (n): G′(ω), G″(ω) ∝ ωn. Thus, the frequency independence of the loss tangent can provide a valid rheological method to determine gelation. For the PSVS65.8-20.0-PE19.4 and PSVS65.8-16.6-PE36.7, the gel-like state (corresponding to a plateau regime of tan δ versus ω) appears after the crossovering point between G′ and G″ from the elastic state in the higher frequency region (G′ > G″). After a limited frequency regime, the tan δ versus ω increases in the PSVS65.8-20.0-PE19.4, meaning entering the more viscous state. However, the change trend of tan δ after the plateau regime for PSVS65.8-16.6PE36.7 is not available due to the limited experimental conditions. From the above analysis, it can be seen that for the PSVS65.8-20.0-PE19.4 and PSVS65.8-16.6-PE36.7 there is the transition of elastic state → gel-like state → viscous state when the angular frequency decreases from the higher frequency regime. In contrast, the modulus−frequency curves for the PSVS-gPE copolymers with low branching density (2.3−3.5 branch chains per 100 repeating units of the backbone) display behaviors markedly different from the comb-like homopolymers7,9 (Figure 7). First, the storage modulus (G′) for these PSVS-g-PE copolymers is larger than the loss modulus (G″) over the entire measured frequency regime. At low frequencies, the G′ levels off, while the G″ is significantly smaller than the G′. Similar rheological features were observed for cross-linked polymer networks at the gel point 42 and for linear polyisoprenes with fillers.43 So this rheological behavior in the PSVS-g-PE copolymers with low branching density can be ascribed to form effective physical cross-linked networks because of the microphase-separated structure at the melt state. Very interestingly, the applicability of time−temperature superposition (TTS) principle for these comb-like copolymers with the low branching density strongly depends on the length of PE branch (or the volume fraction of PE). Although the microphase-separated structure could be observed in all these four samples (Figures 1 and 3), the TTS failure was only observed in the modulus and tan δ curves for PSVS52.7-3.5PE4.9 (lamellar-like structure) and PSVS54.4-2.7-PE10.7 (long G

DOI: 10.1021/acs.macromol.5b01335 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

the valuable advice and technical assistance with the AFM measurement and Prof. Yongfeng Men of CIAC-CAS for valuable advice with the SAXS data analysis. The authors also thank Prof. Zhonghua Wu and Dr. Guang Mo of BSRF for assistance during SAXS measurements.

can explain the typical network-like (or solid-like) rheological behavior of comb-like PSVS-g-PE copolymers with low branching density. However, for PSVS-g-PE with the branch chain number of 16.6−24.5 branch chains per 100 repeating units of the backbone, the average number of the C atoms between two adjacent PE branches is just 8−12. In this case, the PS chain segment is difficult to aggregate each other due to the presence of big hindrance around PS segments coming from PE branch chain. So this kind of comb-like PSVS-g-PE copolymer shows homogeneous phase structure and typical rheological behavior of linear or comb-like homopolymer melts.

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4. CONCLUSIONS The microphase morphology (at the melt state) and linear rheological behavior of PSVS-g-PE copolymers strongly depend on the branching parameters, especially branching density. Comb-like PSVS-g-PE copolymers with low branching density showed a microphase-separated structure (from lamellar-like structure to disorganized sphere-like structure) at the melt state, which resulted in a typical rheological behavior of network-like structure. Moreover, the time−temperature superposition (TTS) principle was not obeyed in the modulus curves for PSVS52.7-3.5-PE4.9 (lamellar-like structure) and PSVS54.42.7-PE10.7 (long tubular structure). In contrast, the PSVS-g-PE samples with high branching density presented homogeneous phase structure at high temperature and normal rheological behavior, similar to linear or comb-like homopolymers. The gel-like state appeared in a limited frequency regime (a plateau regime of tan δ versus ω) during decreasing the frequency from high frequency regime in these comb-like copolymers. We also found that the branch length shows some effect on the final relaxation process, especially the relaxation process of the branch chains. As last, according to the results in this work, it should be mentioned that the comb-like architecture of graft copolymers, compared to block copolymer counterparts, provide more opportunity to adjust microphase separation structure and rheological properties of the copolymers for potential application.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01335. Analytical and spectral characterization data; detailed information about the synthesis, characterization, and analysis of backbone, side chain, and comb-like copolymers (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; Tel +86 (0) 431 85262004; Fax +86 (0) 431 85262827 (T.T.). *E-mail [email protected]; Tel +86 (0) 431 85262516; Fax +86 (0) 431 85262969 (D.X.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (51233005, 21374114, and 21274152). The authors thank Prof. Lin Li of Beijing Normal University for H

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DOI: 10.1021/acs.macromol.5b01335 Macromolecules XXXX, XXX, XXX−XXX