Nanocrystallization-locked Network of Poly(styrene-b-isobutylene-b- styrene)-g-Polytetrahydrofuran Block Graft Copolymer

Poly(styrene-b-isobutylene-b-styrene) triblock copolymer (SIBS), a kind of thermoplastic elastomer with biocompatibility and biostability containing fully saturated soft segments, could be synthesized via living cationic copolymerization. A novel poly[(styrene-comethylstyrene)-b-isobutylene-b-(styrene-co-methylstyrene)]-g-polytetrahydrofuran (M-SIBS-g-PTHF) block graft copolymer was prepared to increase the polarity and service temperature of SIBS by grafting polar PTHF segments onto SIBS. A series of the above block graft copolymers with average grafting numbers from 2 to 6 and molecular weights of PTHF branches ranging from 200 g·mol−1 to 4200 g·mol−1 were successfully synthesized via living cationic ring-opening polymerization of tetrahydrofuran (THF) coinitiated by AgClO4. The introduction of PTHF branches led to an obvious microphase separation due to thermodynamic incompatibility among the three kinds of segments of polyisobutylene (PIB), polystyrene (PS) and PTHF. Moreover, the microphase separation promotes the rearrangement of PTHF branches to form the nanocrystallizationlocked physically cross-linked network after storage at room temperature for 2 months, leading to insolubility of the copolymers even in good solvents. The melting temperature and enthalpy of PTHF nanocrystallization locked in hard domains of M-SIBS-g5-PTHF-1.1k block graft copolymer increased remarkably up to 153 °C and 117.0 J·g−1 by 23 °C and 11.6 J·g−1 respectively after storage for long time. Storage modulus (G') is higher than loss modulus (G'') of M-SIBS-g-PTHF block graft copolymer at temperatures ranging from 100 °C to 180 °C, which is much higher than those of the SIBS triblock copolymer. To the best of our knowledge, this is the first example of high performance M-SIBS-g-PTHF block graft copolymers containing segments of PIB, PS and PTHF with nanocrystallization-locked architecture.


INTRODUCTION
Poly(styrene-b-isobutylene-b-styrene) triblock copolymer (SIBS), a kind of thermoplastic elastomer (TPE), composed of polyisobutylene (PIB) as soft segments (SSs) and polystyrene (PS) as hard segments (HSs), could be synthesized via living cationic sequential block copolymerization of isobutylene (IB) and styrene (St). [1] It has been widely used in biomedical field including endovascular devices, drug carriers, coronary stent, microshunt to treat glaucoma, and stick-to-skin pressure sensor owing to its outstanding biocompatibility and biostability. [2−7] However, thrombosis and infection might occur due to its hydrophobicity on SIBS surface. [8] Moreover, some specific applications of SIBS, such as permselective barrier elastomers with enhanced moisture transmission capabilities, were restricted due to weak polarity and lack of functional group. [9] The service temperature was limited by the glass transition temperature of HSs (T g,HSs ). It was essential to develop functionalized SIBS to improve its hydrophilicity, polarity and T g,HSs .
Three approaches have been used for the synthesis of modified SIBS so far. Polar SIBS could be synthesized via living cationic sequential block copolymerization of IB with polar St derivative monomer, such as p-tert-butyldimethylsilyloxy)styrene (TBDMS), p-tert-butoxystyrene (tBuOS), and 4-[2-(tert-butyldimethylsiloxy)ethyl]styrene (TBDMES). [10−12] Phenolic hydroxyl functionalized SIBS, e.g. poly(hydroxystyrene-b-isobutylene-b-hydroxystyrene), was prepared by the hydrolysis of poly(TBDMS-b-IB-b-TBDMS) or poly(tBuOS-b-IB-b-tBuOS) triblock copolymer. Furthermore, acetylated derivative of SIBS was further synthesized via quantitative acetylation of the phenolic hydroxyl functionalized SIBS. [10−12] Both phenolic hydroxyl functionalized and acetylated SIBS could be used as polymer drug carrier for paclitaxel-eluting coronary stent and improved drug-polymer miscibility, relative to SIBS. [10] Phenolic hydroxyl functionalized random copolymers of IB with vinyl phenol, prepared via cationic co-polymerization of IB with polar comonomers (4-acetoxystyrene or 4-tert-butoxystyrene), exhibit an excellent selfhealing property. This kind of PIB with phenol side groups could be used as a good matrix for the homogeneous dispersion of silica nanoparticles. [13] Chemical modification is a common method for introducing polar functional groups. [14−19] Sulfonated SIBS membranes at high sulfonation levels possessed a complex threedimensional network and high ion exchange capacity, which is potentially used for fuel cell. [14,15] Cross-linked quaternized SIBS (QSIBS) synthesized by chloromethylated SIBS reacting with amine could be used for composite anion exchange membranes. [17−19] Cationic carboxybetaine ester-modified styrenic thermoplastic elastomers were synthesized by chloromethylated SIBS reacting with methyl 3-(dimethylamino) propionate and exhibit efficient bactericidal ability. Meanwhile, antifouling surface against protein, platelets, erythrocytes, and bacteria could be obtained via hydrolysis of carboxybetaine esters into zwitterionic groups. [16] Copolymerization with another monomer to synthesize block or graft copolymer based on SIBS is a common method for introducing new functionalized segments. [8,9,20−26] SIBS with sec-benzyl chloride end groups prepared via living cationic sequential triblock polymerization of IB and St could be used as macroinitiator for atom transfer radical polymerization (ATRP). [9,20−25] 2,5-Bis[(4-methoxyphenyl)oxycarbonyl] styrene (MPCS) was block-copolymerized from sec-benzyl chloride terminated SIBS with low PS content to generate PMPCS-b-PIB-b-PMPCS triblock copolymers, which formed lamellar structures at moderate fractions and hexagonal coil cylinders at high fractions of rod-like PMPCS blocks. [20,21] SIBS-based pentablock terpolymers containing polyacrylate segments could be synthesized by ATRP of methyl acrylate, nbutyl acrylate, tert-butyl acrylate (tBA), and methyl methacrylate initiated by sec-benzyl chloride terminated SIBS. [22,23] Amphiphilic poly(acrylic acid-b-styrene-b-isobutylene-bstyrene-b-acrylic acid) (PAA-b-PS-b-PIB-b-PS-b-PAA) block copolymers could be synthesized via deprotection of the tertbutyl groups using trifluoroacetic acid or simple heat treatment without dissolving the polymer or further sample cleaning from PtBA-b-PS-b-PIB-b-PS-b-PtBA. [22,24] (PAA-b-PS-b-PIB) 2star-PAA and (PS-b-PIB) 2 -star-PAA star terpolymers also could be obtained by the above similar method and represented different microdomain structures containing highly ordered cylinders, lower ordered spheres, and gyroid structures with an increase in PAA content. [9,25] A biocompatible and nontoxic slippery surface could be fabricated via combination of photografting polymerization of 1H,1H,2H,2H-perfluorooctyl methacrylate (FMA) on the SIBS matrixes to generate a coarse morphology and then infusing with fluorocarbon liquid. Moreover, compared to SIBS, this slippery surface of fluorocarbon liquid-infused SIBS-g-PFMA brushes can effectively reduce thrombosis and bacterial infection against E. coli and S. aureus. [8] SIBS-g-polycarboxylate graft copolymers acted as intermediate to create nuclease-modified SIBS via reaction with deoxyribonuclease and ribonuclease, which could availably hinder bacterial adhesion and biofilm formation. [26] Polytetrahydrofuran (PTHF), an aliphatic saturated polyether, could be synthesized via living cationic ring-opening polymerization (CROP) of THF. PTHF has been widely applied due to some advantages such as favorable biological inertness, biocompatibility, breathability, moisture resistance, elasticity at low temperature, flexibility, impact resistance, chemical resistance, and hydrolytic stability. [27,28] PTHF is also commonly introduced into commercially available TPEs for high mechanical properties, [29−31] gene and drug delivery therapy. [32−36] PTHF grafted copolymers displaying obvious microphase separation between macromolecular backbone and PTHF side chains had been synthesized via combination of living CROP and grafting-onto method and reported in our previous works. [33−42] Compared to the unconfined free PTHF segments, the crystallization of PTHF branches in PTHF-grafted copolymers was confined, and the crystallization degree and spherulitic growth rate of PTHF branches decreased with an increase in grafting density or an decrease in the molecular weight of PTHF segments in branches. The melting temperature and enthalpy of PTHF crystallization improved with an increase in the molecular weight of PTHF segments.
The aim of this work is to introduce polar PTHF chains into SIBS backbone and to provide a means of developing novel polar SIBS-based copolymers with polyether. A series of poly[(styrene-co-methylstyrene)-b-isobutylene-b-(styrene-comethylstyrene)]-g-PTHF (M-SIBS-g-PTHF) block graft copolymers with different PTHF lengths (M n,PTHF ) and grafting numbers (G N ) in every M-SIBS backbone were synthesized for the first time via sequential living cationic block copolymerization of IB, St and p-methylstyrene (p-MSt), nucleophilic substitution and living CROP of THF by using brominated M-SIBS (BM-SIBS) as a macroinitiator and silver perchlorate (AgClO 4 ) as a coinitiator. The microphase separation improved after grafted with PTHF polymer. The degree of microphase separation improved with decreases in M n,PTHF and G N . Compared to M-SIBS, nanocrystallization-locked network endowed M-SIBS-g-PTHF block graft copolymers with an increased glass transition of HSs (T g,HSs ) and melting temperature of PTHF crystallization (T m,PTHF ), which is meaningful for improving service temperature of SIBS TPE materials.

Synthesis of M-SIBS Triblock Copolymer
Living cationic block copolymerizations were performed at -80 °C under a dry nitrogen atmosphere in a three-necked round-bottom Schlenk bottle equipped with a mechanical stirrer. The M-SIBS triblock copolymers were produced by sequential addition of the monomers from a small molecular initiator of DCC starting with IB followed by the addition of a prechilled monomer mixture of p-MSt and St. Specially, a 2 L flask was charged with 0.60 mol of IB, 3.45×10 −3 mol of DCC, and 550 mL of n-Hex/CH 2 Cl 2 (60/40, V/V) solvent mixtures. 1.13×10 -2 mol of FeCl 3 with 1.58×10 −2 mol of iPrOH were added into the reaction mixture to start IB polymerization under stirring. After the polymerization of IB for 5−10 min, 60 mL of a prechilled mixture solution containing 0.055 mol of St and 0.073 mol of p-MSt in n-Hex/CH 2 Cl 2 (60/40, V/V) was added into reaction flask. After the procession of block copolymerization for 40 min, polymerizations were terminated by prechilled ethanol. The triblock copolymers were in ethanol and then washed with ethanol for three times. The copolymer products were dried in a vacuum oven at 40 °C to a constant weight, expressed as M-SIBS.

Synthesis of BM-SIBS Macroinitiator
Bromination was carried out in a 1 L closed glass reactor by a teflon plug seal and two ground-in glass stoppers equipped with a mechanical stirrer. The sample of M-SIBS triblock copolymer was dissolved in distilled n-Hex to get a 10 wt% solution. This solution was poured into the three-necked reaction flask in ice-water bath, followed by injecting 0.5 mL of bromine per 10 g of polymer. The flask was transferred to oil bath at 65 °C and the reaction was started by ABVN addition. After 1 h of reaction time, the reaction was terminated by adding a 4.8 wt% NaOH solution, washed with ethanol for three times and stirred for 10 min. The polymer was precipitated in excess ethanol and dried in a vacuum oven at 40 °C to a constant weight, expressed as BM-SIBS.

Synthesis of M-SIBS-g-PTHF Block Graft Copolymer
M-SIBS-g-PTHF was synthesized via living CROP of THF with BM-SIBS (M n =16100 g·mol −1 , PDI=1.22, HS mol%=19%)/AgClO 4 initiating system. Specially, 3.8 g of BM-SIBS and 500 mg of AgClO 4 were added into a prechilled mixture of CH 2 Cl 2 (120 mL) and THF (120 mL) under nitrogen atmosphere. The living CROP of THF was conducted for 3 h at 0 °C under magnetic stirring for synthesis of M-SIBS-g-PTHF + living chains and then terminated by H 2 O molecules. After 3 h of nucleophilic substitution between M-SIBS-g-PTHF + living chains and H 2 O, reaction mixture was centrifuged by a TG16G table-top high-speed centrifuge at 3000 r·min −1 for 10 min, and then supernatant liquid was precipitated with ethanol and deposited at room temperature until supernatant liquid turned into clarified liquid. Then, supernatant liquid was removed. The precipitate was washed by n-Hex to remove unreacted BM-SIBS macroinitiator, followed by drying in a vacuum at 40 °C to a constant weight repeatedly until the weight of product was nondecreasing.

Characterization
The 1 H-NMR spectra of all the copolymers solution were recorded on a Bruker 400 MHz NMR spectrometer AVANCE III. The copolymers of M-SIBS, BM-SIBS and M-SIBS-g-PTHF were dissolved in deuterated chloroform (CDCl 3 , 40 mg·mL −1 ) with tetramethylsilane (TMS) as an internal standard at 25 °C. The calculation method of structural composition, such as molar content of hard segments containing St and p-MSt units (HS mol%), molar content of p-MSt in hard segments (M mol%), average number of Br (N Br ) along polymer chain, average grafting number (G N ) and average molecular weight (M n,PTHF ) of PTHF branches and content of PTHF (W PTHF,NMR ), is shown in the electronic supplementary information (ESI).
Number-average molecular weight (M n ) and polydispersity index (PDI, M w /M n ) of M-SIBS triblock copolymers were determined by a chromatographic system (Waters) using a Waters 1515 isocratic HPLC pump connected to four Waters Styragel HT2, HT3, HT4 and HT6 columns and equipped with a refractive index (RI, Waters 2414) and a ultraviolet visible light detector (UV, Waters 2489, absorption wavelength was 254 nm) double detectors at 30 °C. M-SIBS triblock copolymers were dissolved in THF (2 mg·mL -1 ) and THF was used as the mobile phase at a flow rate of 1.0 mL·min −1 . The columns were calibrated against the standard polystyrene (PS) samples.
The FTIR spectra of M-SIBS, BM-SIBS and M-SIBS-g-PTHF copolymers were recorded on a FTIR analyzer (Nicolet6700, Nicolet Co., USA) in the wavenumber range from 4000 cm −1 to 400 cm −1 with 32 scans and 4 cm −1 resolution at ambient temperature. The solutions of the above-mentioned copolymers in CH 2 Cl 2 (12 mg·mL −1 ) were dropped onto a pre-dried potassium bromide wafer and the spectra were collected after solvent evaporation.
The dn/dc values of samples were used for calculating the absolute weight-average molecular weight (M w ) measured by gel permeation chromatography coupled with multiangle light scattering (GPC-MALS). The dn/dc values of M-SIBS triblock copolymer and M-SIBS-g-PTHF block graft copolymers were determined by a refractive index (RI, Optilab T-rEX), which was used for calculating the corresponding absolute weight-average molecular weight. The copolymers of M-SIBS and M-SIBS-g-PTHF were dissolved in THF with a series of concentrations (0.25, 0.5, 0.75, 1, 1.25, and 1.5 mg·mL −1 ) and THF was used as the mobile phase at a flow rate of 0.5 mL·min −1 . The slopes of obtained fitting curves were dn/dc values. The results demonstrate tiny difference in dn/dc values between M-SIBS triblock copolymer and M-SIBS-g-PTHF block graft copolymer due to small G N of PTHF branches.
The absolute M w of the copolymers was determined by a chromatographic system (Wyatt Technology) using a LabAlliance series 1500 isocratic HPLC pump connected to four MZ-Gel Styragel 10 2 Å, 10 3 Å, 10 5 Å, and 10 6 Å columns in series and equipped with a refractive index (RI, Optilab T-rEX), four bridge capillary viscometer (VIS, Wyatt ViscoStar-II) and multiangle laser light scattering (MALS, DAWN HELEOS-II, laser wavelength was 658 nm) triple detectors at 35 °C. The MALS signals were calibrated against the narrow polystyrene standard with M w of 3.0×10 5 g·mol −1 and dn/dc value of 0.1845 mL·g −1 was used for PS standards. was injected. The absolute M w of copolymers was calculated by using dn/dc value of 0.14 mL·g −1 . The content of PTHF (W PTHF,GPC ) could be calculated according to Eq. (1): where stands for the absolute weight-average molecular weight of M-SIBS triblock copolymer and stands for the absolute weight-average molecular weight of M-SIBS-g-PTHF block graft copolymers.
The dilute solution of M-SIBS triblock copolymers and M-SIBS-g-PTHF block graft copolymers in toluene with a concentration of 0.2 mg·mL −1 was dropped onto a copper grid with carbon film by pipette. After complete volatilization of solvent, the copper grid with loaded samples was placed in oven at 50 °C for 8 h, followed by being annealed to room temperature. Then it was dyed with ruthenium tetroxide (RuO 4 ) for 20 min at room temperature. The flat surfaces of M-SIBS-g-PTHF block graft copolymers were prepared at −80 °C by using the cryomicrotome (Leica EM UC7, Germany). The micromorphology of copolymers was observed by a transmission electron microscope (TEM, Hitachi HT-7700) at an acceleration voltage of 100 kV. The micromorphology of crystallization in M-SIBS-g-PTHF block graft copolymers was observed by a high-resolution transmission electron microscope (HRTEM, JEOL JEM-3010) at an acceleration voltage of 300 kV.
The dilute solution of M-SIBS triblock copolymers and M-SIBS-g-PTHF block graft copolymers in toluene with a concentration of 0.2 mg·mL −1 was dropped onto a silicon wafer to form film by KW4A spin coater (Beijing SETCAS Electronics Co., Ltd) at 3000 r·min −1 for 60 s. The silicon wafer with loaded samples was put in oven annealed at 50 °C for 8 h, followed by being annealed to room temperature. Then it was observed in tapping mode by Bruker DI ultra-fast probe atomic force microscope (AFM) under N 2 atmosphere.
M-SIBS-g-PTHF block graft copolymers were soaked in cyclohexane for 2 weeks, followed by vacuum freeze drying at −80 °C for 12 h. Then, the surfaces and cross sections of freeze-dried samples were sputter-coated with gold and were observed with a field emission scanning electron microscope (SEM, Hitachi S-4700) at an acceleration voltage of 20 kV and a working current of 10 μA.
Differential scanning calorimeter (DSC) thermograms on M-SIBS triblock copolymer and M-SIBS-g-PTHF block graft copolymers were collected by Q200 thermoanalyzer system (TA Co., USA) with 10 °C·min −1 heating and cooling rate under N 2 atmosphere. Samples were cut into many pieces with weights of 5−10 mg for DSC testing.
The rheological behavior of M-SIBS triblock copolymer and M-SIBS-g-PTHF block graft copolymer was examined using a TA Instruments DHR-2 rheometer with an environmental test chamber (ETC). Dynamic isothermal frequency sweeps were performed using 8 mm diameter parallel geometry plates at different temperatures, at a frequency that ranged from 0.1 Hz to 100 Hz. Dynamic temperature ramps were performed at a frequency of 1 Hz while heated at 3 °C·min −1 .

RESULTS AND DISCUSSION
The synthetic strategy for poly[(styrene-co-methylstyrene)-b-isobutylene-b-(styrene-co-methylstyrene)]-g-polytetrahydrofuran (M-SIBS-g-PTHF) block graft copolymers to modify M-SIBS backbone by introducing PTHF segments is shown in Scheme 1. The synthetic routes include three steps: (1) synthesis of M-SIBS triblock copolymer by living cationic block copolymerization of IB with St and p-MSt; (2) synthesis of BM-SIBS macroinitiator by nucleophile substitution of molecular bromine with benzyl groups on M-SIBS triblock copolymer; (3) synthesis of M-SIBS-g-PTHF block graft copolymers via living CROP of THF using BM-SIBS/AgClO 4 as an initiating system.

Synthesis and Characterization of M-SIBS Triblock Copolymer
The schematic illustration for M-SIBS triblock copolymers is shown in Scheme 1. Living cationic polymerization of IB with DCC/FeCl 3 /iPrOH initiating system could be achieved in n-Hex/CH 2 Cl 2 mixed solvents at −80 °C. Subsequently, the fresh mixed solution of St and p-MSt was added for block copoly-  The different amounts and proportions of fresh mixed solution of St and p-MSt were added in the block copolymerization with DCC/FeCl 3 /iPrOH initiating system after almost quantitative consumption of IB in the living cationic poly-merization system, leading to formation of M-SIBS triblock copolymers with different molecular weights (M n =14.0− 16.1 kg·mol −1 ) and copolymer compositions (HS mol%=7%− 20%, M mol%=52%−98%). The GPC traces of triblock copolymers with RI/UV double detectors are shown in Fig. S1 (in ESI). All structural units were reflected in RI detector, while only phenyl groups of styrene and p-MSt units in the copolymer chains could be detected in UV detector. All GPC traces of M-SIBS triblock copolymers by RI/UV double detectors were consistent with each other, which demonstrates that styrene and p-MSt units were introduced to PIB chain ends to form M-SIBS triblock copolymers. All GPC traces of M-SIBS triblock copolymers possessing different structural compositions with RI detector present symmetrical and unimodal molecular weight distribution. Characterization results for M-SIBS triblock copolymers are summarized in Table S1 (in ESI).

Bromination of M-SIBS Triblock Copolymer
Brominated M-SIBS triblock copolymer (BM-SIBS) was prepared by using molecular bromine as bromine source, as shown in Scheme 1. The nucleophilic substitution of molecular bromine with benzyl groups was conducted in the presence of ABVN in n-Hex at 65 °C. The reaction is fast due to direct generation of a mass of bromine radicals. The degrees of bromination of four BM-SIBS triblock copolymers were determined to be 18%, 40%, 42% and 69%, respectively.
The representative FTIR spectra of M-SIBS triblock copolymer and BM-SIBS macroinitiator are shown in Fig. S2 (in ESI). It can be observed that a new characteristic absorbance at 608 cm −1 generated, which is attributed to the stretching vibration of C-Br in benzyl bromide (p-BMSt) groups after bromination. The result preliminarily proved that bromine atom could be successfully introduced to M-SIBS.

Synthesis and Characterization of M-SIBS-g-PTHF Block Graft Copolymers
As shown in Scheme 1, terminal hydroxyl functionalized M-SIBSg-PTHF block graft copolymers were successfully synthesized in CH 2 Cl 2 by using the BM-SIBS with multifunctional benzyl bromine side functional groups as macroinitiators to initiate living CROP of THF in the presence of AgClO 4 at 0 °C to create living M-SIBS-g-PTHF + chains, which were terminated by H 2 O via nucleophilic substitution.
The representative FTIR spectra of BM-SIBS macroinitiator and M-SIBS-g-PTHF block graft copolymer are shown in Fig.  S3 (in ESI). A new characteristic absorbance at 1110 cm −1 is assigned to the stretching vibration of the C-O-C groups in PTHF branches. The characteristic band at 820 cm −1 attributed to the bending vibration from the benzene rings of PS segments became weak due to the restriction arising from PTHF branches linking the p-MSt units. The broad and weak characteristic band at 3445 cm −1 attributed to the stretching vibration from the terminal -OH groups of PTHF branches could be observed in FTIR spectrum of the M-SIBS-g-PTHF + chains. It suggests that M-SIBS-g-PTHF block graft copolymers could be successfully synthesized.
The 1 H-NMR characterization was further performed on a representative M-SIBS-g-PTHF block graft copolymer to evaluate the chemical structure, as shown in Fig. 3 The RI and MALS responsive traces of M-SIBS triblock copolymer and M-SIBS-g-PTHF block graft copolymers are shown in Fig. S4 (in ESI). The signal strength in RI detector is responsible for concentration, while the signal strength in MALS detector is responsible for molecular weight. The molecular weight shifted to higher molecular weight and the molecular weight distribution became broad after grafting    Table S1 (in ESI).

Effect of G N and M n,PTHF on Aggregation Structure of M-SIBS-g-PTHF Block Graft Copolymers
Thermodynamic incompatibility from different segments in copolymers results in microphase separation. Transmission electron microscopy (TEM) was performed on M-SIBS triblock copolymer and M-SIBS-g-PTHF block graft copolymer dyed with RuO 4 to characterize microphase separation structure in copolymers, as shown in Fig. 4. The light gray phases are attributed to the PIB segments. The size of hard domains increased after being grafted with PTHF polymer due to visible dyed PS and associated crystallisable PTHF segments (dark black phase). In contrast, M-SIBS-g-PTHF block graft copolymer was invisible without dyeing, as shown in Fig. S6(a) (in ESI). The TEM image of blank copper grid is shown in Fig. 4(c) and Fig. S6(b) (in ESI) for comparison.
To investigate the micromorphology of PTHF domains, HRTEM was applied to observe thin film formed from dilute solution of M-SIBS-g-PTHF block graft copolymer. The HRTEM images at different magnifications and fast Fourier transform (FFT) image are shown in Fig. 5. Clear microphase separation phenomenon at the nanoscale could be observed at relatively low magnification (Fig. 5a), which was different from foregoing TEM image dyed with RuO 4 (Fig. 4b) due to invisible PS segments without dyeing. Regular lattice diffraction lines with lattice diffraction spacing of 0.222 nm appeared at relatively high magnification (Fig. 5b), demonstrating that PTHF nanocrystallization indeed existed. Moreover, diffraction points from PTHF nanocrystallization and dispersion ring from amorphous PIB and PS segments could be simultaneously observed in FFT image (Fig. 5c) of M-SIBS-g-PTHF block graft copolymer. These results proved the existence of PTHF   nanocrystallization in bulk of M-SIBS-g-PTHF block graft copolymers.
The phase images of surfaces in M-SIBS triblock copolymer and M-SIBS-g-PTHF block graft copolymers were obtained by AFM to further study microphase separation structure. As shown in Fig. S7 (in ESI), clear microphase separation phenomenon at the nanoscale could be observed. The phase images by AFM can be used for calculating the separated size of domains. [44] The phase profiles of the topographs were presented in Fig. S7 (in ESI). After grafted with PTHF segments, strong separated peaks translated into weak multiplet due to introduction of tertiary component. Four sizes of separated domains formed on the surfaces of M-SIBS triblock copolymer and M-SIBS-g-PTHF block graft copolymers; from a to d, they were 167, 67, 83, and 103 nm, respectively. The separated size of domain structure is related to structural composition of copolymers. [44] The relationship between separated size of domain structure and G N , M n,PTHF is shown in Fig. 6. The separated size significantly decreases after grafted with PTHF polymer, indicating that microphase separation degree was promoted by grafting PTHF segments. The separated size increases with increases in G N and M n,PTHF , indicating that the size of domains containing PTHF and PS increases due to more PTHF branches incorporated into the SIBS backbone.
The height sensor images of surfaces in M-SIBS triblock copolymer and M-SIBS-g-PTHF block graft copolymers obtained by AFM were used to investigate the influence of G N and M n,PTHF on the flatness of surfaces. As shown in Fig. 7, the height difference (ΔH) significantly decreased after grafted with PTHF polymer. The roughness (R a ) of surfaces in M-SIBS triblock copolymer and M-SIBS-g-PTHF block graft copolymers are shown in Fig. 8. All the R a values were less than 1.5 nm before and after grafted with PTHF polymer due to small G N of 2−6 and M n,PTHF of 200−4200 g·mol −1 . There was a tiny decrease in R a with increases in G N and M n,PTHF .

Nanocrystallization-locked Network in M-SIBS-g-PTHF Block Graft Copolymers
The samples of M-SIBS-g-PTHF block graft copolymers after storage at room temperature for 2 months became insoluble. It can be seen from Fig. 9(a) that M-SIBS-g-PTHF block graft copolymer is insoluble in cyclohexane. Compact physically cross-linked network was supposed to exist in M-SIBS-g-PTHF block graft copolymers due to PTHF crystallization locked in hard domains. To prove this hypothesis, M-SIBS-g-PTHF block graft copolymers were soaked in cyclohexane for 2 weeks, followed by freeze drying. The surfaces and cross sections after swelling of M-SIBS-g-PTHF block graft copolymers with different PTHF contents were observed by SEM, as shown in Figs Fig. S8c in ESI). The tightness and integrity of physically cross-linked network in the surfaces decreased with a decrease in PTHF content. Moreover, it is clear from cross section after swelling that only surface with certain depth was immersed and no change happened inside, indicating the formation of compact physically cross-linked network due to PTHF nanocrystallization locked in M-SIBS-g-PTHF block graft copolymers.
To prove the existence of PTHF nanocrystallization in M-SIBS-g-PTHF block graft copolymers, ultrathin sections without dyeing were observed by TEM. As shown in Fig. 10, clear microphase separation could be observed in M-SIBS-g-PTHF block graft copolymers due to PTHF nanocrystallization (dark black), which was clearly different from Fig. S6(a) (in ESI). When M n,PTHF decreased from 3.2k to 1.4k, the size of PTHF nanocrystallization decreased due to the short PTHF segments against crystallization. Therefore, the PTHF nanocrystallization in M-SIBS-g 5 -PTHF-1.1k became more unconspicuous due to the lower M n,PTHF compared with M-SIBS-g 4 -PTHF-1.4k. Moreover, PTHF nanocrystallization in M-SIBS-g-PTHF block graft copolymer was also observed by HRTEM. The HRTEM images of ultrathin section without dyeing are shown in Figs. 11(a) and 11(b). Clear microphase separation phenomenon at the nanoscale and regular lattice diffraction lines still existed, but the lattice diffraction spacing increased to 0.241 nm. Diffraction points and dispersion ring also could be simultaneously observed in FFT image (Fig. 11c), indicating that PTHF nanocrystallization was stable in M-SIBS-g-PTHF block graft copolymer. The schematic diagram of the formation of nanocrystallization-locked physically cross-linked network was proposed on the basis of above results, as shown in Fig. 12. Nanocrystallization created and locked in hard microphase domains due to crystallisable PTHF branches embedded in the hard PS domains.
DSC was applied to investigate the transformation of HSs after grafted with PTHF polymer, as shown in Fig. 13(a). Two glass transition temperatures appeared during heating, suggesting phase separation behavior between SSs and HSs. No glass transition temperature of PS in HSs (T g,HSs ) was observed after grafted with PTHF polymer, which may be attributed to crystallisable PTHF segments in cross-linked network confining segmental mobility of PS segments. A broad endothermic peak was observed during heating, which was attributed to the melting temperature of PTHF (T m,PTHF ) in crosslinked network. The T m,PTHF shifted to higher temperature with increases in G N and M n,PTHF , which was consistent with other PTHF-grafted copolymers, [32,33,40] as shown in Fig. 13(b). The T m,PTHF and melting enthalpy increased from 23 °C and 11.6 J·g −1 in original soluble M-SIBS-g 5 -PTHF-1.1k block graft copolymer ( Fig. S9 in ESI) to 153 °C and 117 J·g −1 , respectively, in nanocrystallization-locked M-SIBS-g 5 -PTHF-1.1k block graft copolymer network. Compared to the T m,PTHF of 7.0−43.2 °C in neat PTHF and PTHF-based block or graft copolymers, [28,31,37,38,42] the T m,PTHF in M-SIBS-g-PTHF block graft copolymer networks increased to above 140 °C due to PTHF nanocrystallization locked in cross-linked network containing hard PS domains. In order to confirm the formation of physically cross-linked network in M-SIBS triblock copolymer and M-SIBS-g-PTHF block graft copolymer, the rheological behavior was measured by rheometer. The temperature ramp of M-SIBS triblock copolymer at a frequency of 1 Hz is shown in Fig. 14(a). The physically cross-linked network formed by microphase separation began to dissociate at 67 °C, corresponding to T g,HSs (Fig. 13a). Viscous transition appeared at 130 °C when the network completely dissociated. G″ versus G′ plots for M-SIBS triblock copolymer by frequency sweeps at various temperatures but at the same frequency are shown in Fig. 14(b), which can be generally used to investigate structural changes. [45−47] If the G″ versus G′ plots measured at different temperatures are superimposable, the material exhibits no structural change in the examined temperature range. Obvious structural change occurred at the range from 110 °C to 130 °C, fur-ther verifying the disappearance of network. The temperature ramp of M-SIBS-g 5 -PTHF-1.1k block graft copolymer at a frequency of 1 Hz is shown in Fig. 14(c). When the temperature increased from 100 °C to 170 °C, G′ slightly decreased M-SIBS-g-PTHF PBLG-g-PTHF [32] PVAc-g-PTHF [33] HPC-g-PTHF [40] Fig. 13 (a) DSC thermograms recorded during heating for M-SIBS triblock copolymer and M-SIBS-g-PTHF block graft copolymers. (b) Ashby plot of T m,PTHF in M-SIBS-g-PTHF block graft copolymers and other PTHF-grafted copolymers reported in literature. [32,33,40] while G″ sharply decreased. G′ is higher than G″ in the range from 100 °C to 170 °C, indicating that the integrity of network was maintained. G″ versus G′ plots for M-SIBS-g 5 -PTHF-1.1k block graft copolymer by frequency sweeps at various temperatures but at the same frequency are shown in Fig. 14(d). The differences between G′ and G″ broaden with an increase in temperature. Therefore, no obvious structural change occurred at 100 °C and 150 °C, while structural change occurred at 180 °C due to the melt of PTHF crystallization.

CONCLUSIONS
A series of M-SIBS-g-PTHF block graft copolymers with various average grafting numbers (G N ) and molecular weights of branches (M n,PTHF ) were synthesized via combination of living cationic block copolymerization of IB, St and p-MSt, nucleophile substitution of M-SIBS with molecular bromine, and living CROP of THF. M-SIBS-g-PTHF block graft copolymers displayed clear microphase separation. The microphase separation improved after grafted with PTHF segments. The size of HSs increased and the separated size of microphase in M-SIBS-g-PTHF block graft copolymers decreased after grafted with PTHF segments. The separated size of microphase slightly increased with increases in G N and M n,PTHF due to PTHF. The roughness (R a ) decreased after grafted with PTHF segments and exhibited a gradually dropping trend with increased G N and M n,PTHF . PTHF branches rearranged due to microphase separation to form nanocrystallization locked in network after storage at room temperature for 2 months. The melting temperature and enthalpy of PTHF nanocrystallization locked in hard domains of M-SIBS-g 5 -PTHF-1.1k block graft copolymer increased remarkably upto 153 °C and 117.0 J·g −1 by 130 °C and 106 J·g −1 respectively after storage for long time. The movement of PS segments was confined by PTHF nanocrystallization locked in network, which could effectively improve the service temperature of SIBS TPE materials.

Electronic Supplementary Information
Electronic supplementary information (ESI) is available free of charge in the online version of this article at https://doi.org/10.1007/s10118-021-2536-9.

ACKNOWLEDGMENTS
This work was financially supported by the China Petrochemical Corporation and the Fundamental Research Funds for the Central Universities (Nos. XK1802-2 and XK1802-1).