Order-order transition induced by mesophase formation in a novel type of diblock copolymers based on poly(isobutyl methacrylate) and poly[2,5-di(isopropyloxycarbonyl)styrene]

Abstract

Novel diblock copolymers based on poly(isobutyl methacrylate) (PiBMA) and poly[2,5-di(isopropyloxycarbonyl)styrene] (PiPCS) were designed and prepared via consecutive atom transfer radical polymerization. They had relatively low molecular weight distributions and tunable molecular weights. The molecular characterization of the copolymers was performed with proton nuclear magnetic resonance spectroscopy, gel permeation chromatography, and thermogravimetric analysis. The phase structures and transitions were investigated by differential scanning calorimetry, small- and wide-angle X-ray scattering, and polarized optical microscopy experiments. A PiPCS block with a polymerization degree higher than 168, exhibited a stable hexagonal columnar liquid crystalline (Col_h LC) phase, regardless of the length of PiBMA block. The PiPCS block underwent a coil-to-rod conformational change when it transformed into Col_h LC phase. For the PiBMA-b-PiPCS diblock copolymer, this conformational transition resulted in a topological change from a coil-coil to a rod-coil type structure. With increasing PiPCS fraction, the block copolymers' microphase separated structure changed from an undetermined phase, to a lamellar phase, to a PiBMA columnar phase in the coil-coil system. The original microphase separated structure observed with coil PiPCS block could evolve into a lamellar morphology, with a significantly increased long period when PiPCS transformed into Col_h LC phase. Thereafter it served as a rod in the block copolymers studied. This microphase separated structure transition was triggered by the coil-to-rod (isotropic-to-LC) transition and was irreversible due to the fact that the LC phase of PiPCS block remained at higher temperatures until decomposition.

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Introduction

Block copolymers (BCPs) can be constructed by covalently connecting two or more dissimilar polymer segments together. The immiscibility of different blocks will induce microphase separation into ordered structures on the length scale of 5–100 nm.^1,2 For coil-coil diblock copolymers, which are composed of two blocks exhibiting similar flexible conformations, the phase behavior is mainly governed by the overall degree of polymerization N = N_A + N_B, the composition f_A = N_A/N and the energy of mixing the A and B segments, characterized by the Flory-Huggins interaction parameter χ. The mechanisms leading to equilibrium microphase morphologies, including spheres, hexagonally packed cylinders, double gyriod, and lamellae, are well understood.^3,4 In addition, χ is inversely related to temperature. One may observe an order-disorder transition (ODT) corresponding to the loss of microphase segregation or an order-order transition (OOT) corresponding to a thermotropic phase change in microphase morphology when a sample is heated.

When liquid crystalline polymers (LCP), which tend to self-organize into ordered structures on the length scale of 1–10 nm, are introduced into BCPs, complex nanostructures on different length scales in polymer systems can be realized. The molecular design and synthesis of liquid crystalline BCPs with well-defined structure and narrow molecular weight distribution, as well as the interplay between the liquid crystalline (LC) phase formation and microphase separation, have been investigated for several decades because of the unique advantage in constructing hierarchical nanostructures and manipulating functionality within them.^5–12 The liquid crystalline block candidates are frequently focused on side-chain liquid crystalline block polymers (SCLCPs) or rod-like conjugate polymers. As for SCLCPs, the smectic layer formation originating from the self-assembly of side-chain mesogens made the BCPs tend to form into lamellar^13,14 or cylinder-like morphologies.^9,15 Other morphologies easily frustrate the smectic LC structure, such as spherical microdomain with highly curved interface, and thus only nematic phase can be observed.^16,17 The LC to isotropic (LC-I) transition of an LCP block can also trigger the microphase transition of BCPs.^13,18,19 For example, Sänger et al. reported a body-centered cubic spheres-hexagonally packed cylinders transition in the morphology of a block copolymer, accompanied by the liquid crystalline phase formation of the LC block.²⁰

As for rod-like conjugate polymers, since the liquid crystalline behavior originates from the parallel alignment of anisotropic rod-like polymers themselves, the microphase diagram is more complicated because of both the LC behavior and the conformational asymmetry of the two blocks. Novel nanoscale morphologies which are different from those of traditional coil-coil diblock copolymers have been investigated recently.^6,21–28 Lamellar morphologies with a planar interface, such as zig-zag, arrow-headed, and perforated lamellae, which accommodate the parallel alignment of rod-like blocks have been reported in some rod-coil BCP systems. In this way, the LC aggregation of the rod-like block dominates the nanostructure formation. Upon decreasing the interaction amongst rod blocks, thermal transitions of rod-coil BCPs can be detected. SmA-N-I and SmA-I phase transitions can be found in PS-b-PPV, in which a PPV block was decorated by alkoxy chains via chemical modification around the PPV rod.^29–31 As for core-shell-like rod-coil block copolymers, with the microphase separation dominating the microphase structure formation, the curved interface of a spherical microdomain would decrease the LC order of rod blocks from Col_h to columnar nematic phase.³²

When the rod block exhibited conformational change, for example, poly(γ-benzyl-L-glutamate) (PBLG) exhibits both rod-like behavior because of an α-helical secondary structure and coil conformation, OOT or ODT could be observed based on the conformational change triggered by varying the temperature or by varying the solvent composition. Borsali and coworkers have investigated the solution properties of a poly(styrene-d₈)-b-poly(γ-benzyl-L-glutamate) system and found the conformational transition resulted in a topological change from a rod-coil to a coil-coil type structure.³³ Klok et al.^23,34 have reported the OOT of peptide-based diblock oligomers from a double hexagonal (PS10-b-PBLG20) or a columnar hexagonal (PS10-b-PBLG40, PS10-b-PZ-Lys80) to a lamellar β-sheet morphology by increasing the temperature.

All the LC polymers discussed above exhibited phase transition from LC to an isotropic state when the temperature was increased. For mesogen-jacketed liquid crystalline polymers poly{2,5-bis[(4-butoxyphenyl)oxycarbonyl]styrene} and poly{2,5-bis[(4-hexoxyphenyl)oxycarbonyl]styrene} which displayed an isotropic phase at lower temperatures and a liquid crystalline phase at higher temperatures, the phase behavior is different from those of regular LC polymers.^35–37 Recently, similar LC phase behavior of poly[2,5-di(isopropyloxycarbonyl)styrene] (PiPCS) with no traditional mesogen was reported.^38,39 The polymer can melt into an isotropic state, first above its glass transition temperature (T_g) of 88 °C, and then a Col_h LC phase is developed and this becomes stable at temperatures above 160 °C, when the degree of polymerization (DP) reaches a critical value. No obvious extinction of the birefringence can be observed before the polymers decompose (onset temperature > 300 °C).³⁹ The LC phases of these polymers are formed via a supramolecular assembly of the polymer chains, which possess an overall cylindrical shape because of the lack of spacers between the polymer backbones and the side groups. As a new type of building unit to create block copolymers (BCPs), PiPCS is extremely attractive due to such unique characteristic LC properties. With the transition from amorphous state to LC phase due to the increase in temperature, a unique OOT should be detected. Two things should be considered. One is how the confined nano-environment offered by BCP microphase separation influences the supramolecular LC formation. The other is how the LC formation influences the microphase structure of BCPs.

A series of diblock copolymers poly(isobutyl methacrylate)-b-poly[2,5-di(isopropyloxycarbonyl)styrene] (PiBMA-b-PiPCS) with varying molecular weights (MWs) and relatively narrow MW distributions were synthesized via consecutive atom transfer radical polymerization. PiBMA with relatively low T_g offers a soft environment for the LC formation of PiPCS. The morphology of the diblock copolymers would depend both on the relative block-lengths and on the conformation of the PiPCS segment. Based on the above consideration, the morphologies of three special diblock copolymers were studied in detail. These copolymers contained different PiPCS contents (PiBMA313–PiPCS168, PiBMA313–PiPCS267, and PiBMA313–PiPCS594). According to the experimental results, we found that the original microphase separated structure observed with coil PiPCS block indeed evolved into a lamellar morphology with a significantly increased long period when PiPCS formed Col_h LC phase in the lamellar phase. We observed a transition from an unconfirmed phase to a lamellar phase, a transition from a lamellar phase to another lamellar phase, and a transition from PiBMA columnar phase to a lamellar phase when PiBMA313–PiPCS168, PiBMA313–PiPCS267, and PiBMA313–PiPCS594 were subjected to the first heating, respectively.

Results and discussion

Polymerization

The block copolymers were synthesized by sequential atom transfer radical polymerization of isobutyl methacrylate and 2,5-di(isopropyloxycarbonyl)styrene (Scheme 1), see ESI for details.† High degree of chain-end functionality and narrow polydispersity of the macroinitiator PiBMAm-Br were critical to the success of the subsequent addition reaction. Therefore, we stopped the polymerization at relatively low conversion to reduce undesired side reactions. Fig. 1 shows GPC chromatograms of PiBMA313-Br and three representative block copolymers. Compared to PiBMA313-Br, the GPC curves of block copolymers shifted to higher molecular weights. In addition, they were symmetrically monomodal and with narrow MW distribution, indicating that no homo-PiPCS formed during polymerization. To remove any remaining PiBMAm-Br, the block copolymers were extracted by n-hexane, a good solvent for PiBMA block and a nonsolvent for PiPCS block, for 24 h. Unreacted monomers could be completely eliminated by repeatedly dissolving polymers in THF and precipitating them in methanol, because of the good solubility of the monomer in methanol. The detailed data is listed in Table 1.

Scheme 1 Synthetic procedure of poly(isobutyl methacrylate)-b-poly [2,5-di(isopropyloxycarbonyl)styrene]. The copolymer is abbreviated to PiBMAm–PiPCSn, where m and n denote the degree of polymerization of PiBMA and PiPCS, respectively.

Table 1 Properties of the resultant block copolymers poly(isobutyl methacrylate)-b-poly[2,5-di(isopropyloxycarbonyl)styrene]

Sample ID	PiBMA			PiPCS	Diblock copolymers		T d	fPiPCS^a	LC^d
	M n	M n	M w/Mn^b	M n	M n	M w/Mn^b
a Molecular weights of PiBMA and PiPCS as well as the PiPCS segment volume fraction were estimated using ^1H NMR. b Number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (Mw/Mn) were obtained by gel permeation chromatography, calibrated against a series of monodispersed polystyrene standards. c Temperature at which the weight loss of polymers reached 5% at a heating rate of 20 °C min^−1 under nitrogen. d Determined by polarized optical microscopy.
PiBMA118-PiPCS212	16,800	16,500	1.19	58,400	70,600	1.24	253	77.3	Yes
PiBMA118–PiPCS400				110,400	142,900	1.35	315	86.6	Yes
PiBMA268-PiPCS32	39,300	37,400	1.13	8,800	53,400	1.19	251	18.0	No
PiBMA268–PiPCS391				126,700	107,400	1.26	307	75.9	Yes
PiBMA313-PiPCS83	44,400	42,000	1.11	22,800	89,400	1.20	263	32.4	No
PiBMA313–PiPCS168				46,400	104,500	1.11	280	50.4	Yes
PiBMA313–PiPCS200				55,300	104,700	1.24	284	54.8	Yes
PiBMA313–PiPCS267				73,800	110,300	1.19	294	61.8	Yes
PiBMA313–PiPCS504				139,300	138,400	1.17	300	75.3	Yes
PiBMA313–PiPCS594				164,100	158,700	1.30	302	78.2	Yes
PiBMA313–PiPCS602				166,400	162,000	1.26	306	78.5	Yes

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Fig. 1 GPC curves of PiBMA313-Br macroinitiator and three representative diblock copolymers.

The copolymer composition was determined by nuclear magnetic resonance spectroscopy (¹H NMR, 400 MHz) with acetone-d6 as the solvent. The ¹H NMR spectra of PiBMA168-Br, PiPCS162, and the block copolymer PiBMA268–PiPCS391 are shown in Fig. 2. The DPs of PiBMA and PiPCS in the block copolymers were calculated from the ¹H NMR spectra with the following equation:

DP_PiBMA = (I_3.7–3.8 × 5)/(I_7.40–7.42 × 2) (1)

DP_PiBMA/DP_PiPCS = [(I_3.7–3.8 + I_3.1–3.7) × 2 − I_4.5–5.4]/(I_4.5–5.4 × 2) (2)

where I_3.7–3.8, I_7.40–7.42, I_3.1–3.7, and I_4.5–5.4 were the integration of the NMR peaks at 3.7–3.8 ppm (–O–CH₂– in the iBMA units), 7.40–7.42 ppm (the phenyl protons of the benzyl 2-bromoisobutyrate initiator), 3.1–3.7 ppm (–CH– in the PiPCS backbones), and 4.5–5.4 ppm (–O–CH– in the iPCS units), respectively. DP_PiBMA and DP_PiPCS were the DPs of PiBMA and PiPCS in the block copolymers. With this approximate data and the densities of PiBMA and PiPCS, the volume fractions of PiPCS (f^PiPCS) were calculated for the diblock copolymers (Table 1). The big differences between experimental M_n values acquired by GPC and those calculated from the ¹H NMR spectra were due to the differences of the hydrodynamic volumes of these block copolymers versus linear polystyrene standards.

Fig. 2 ¹H NMR spectra of PiBMA268, PiPCS162, and a representative block copolymer PiBMA268–PiPCS391 (acetone-d6, 400 MHz).

Mesomorphic properties

The copolymers had relatively good thermal stability and the 5% weight loss temperature was about 300 °C under N₂ atmosphere. Because of the relatively poor thermal stability of the PiBMA block, the development of the liquid crystalline phase was carried out at 170 °C under N₂, to rule out the possible influence of thermal decomposition of PiBMA on the microphase separation of the copolymers. All the DSC thermograms were featureless except for glass transitions (Fig. 3). The subsequent heating thermograms of the block copolymers at 20 °C min⁻¹ showed a broad transition due to two overlapped glass transitions located at around 68 and 88 °C, belonging to the PiBMA and PiPCS blocks, respectively. This demonstrated that the block copolymers remained microphase separated. For thermograms of the other block copolymers with PiPCS contents of more than 70%, only one glass transition of the PiPCS block was discernable, which was due to the low PiBMA content.

Fig. 3 Differential scanning calorimetric thermograms of three representative diblock copolymers and homopolymers PiBMA313 and PiPCS168 during the second heating scan at a heating rate of 20 °C min⁻¹.

Liquid crystalline phases of the PiPCS domains obtained were first studied by polarised optical microscopy (POM) observation. When DP of PiPCS segment was higher than 168, the block copolymer exhibited birefringent textures, indicating liquid crystalline ordering of the PiPCS block. No relationship between the liquid crystallinity and the length of PiBMA block was found in the three series of diblock copolymers. POM images exhibited focal-conic fan-like textures at 170 °C for samples that were isothermally annealed for several hours (Fig. 4a). For the identification of the ordered structures developed in the block copolymers, 1D WAXD experiments were carried out with solution-cast samples. Fig. 4b and 4c illustrate the two sets of temperature-variable 1D WAXD patterns of sample PiBMA313–PiPCS168 in a 2θ range of 2–30° obtained during the first heating and the subsequent cooling procedures. In Fig. 4b, a low-angle broad scattering halo (2θ = 6.61°) was observed at the starting test temperature of 40 °C. The scattering halo remained broad up to 150 °C, higher than the T_gs of the PiPCS and PiBMA blocks, indicating an isotropic liquid. One narrow reflection peak developed at 160 °C and the intensity of the peak increased gradually with an isothermal treatment at 170 °C (2θ = 6.56°, with a d-spacing value of 1.36 nm). This indicated that an ordered structure on the nanometre scale was developed. This was consistent with the POM result. Higher-order diffractions were visible (the enlarged part in Fig. 4b), and the scattering vector ratio of the diffractions was 1 : √3 : √4, indicating a long-range ordered hexagonal lattice, therefore a Col_h phase. The temperature of entering the LC phase was higher than that of PiPCS homopolymers with approximately the same DP, which could be attributed to the microphase separated structure of diblock copolymers. The WAXD experiment during the subsequent cooling of the copolymer was carried out, with the results shown in Fig. 4c. The evolved reflection peaks (2θ = 6.56°, 11.37°,13.10°) remained, but the scattering intensity became weaker with decreasing temperature, possibly due to decreased difference in densities (and thus electron densities) of the two blocks with decreased temperature. Furthermore, the 1D WAXD experiments of other copolymers were also performed, and similar phenomena were observed for diblock copolymers with DP of PiPCS segment not less than 168 were observed. On the basis of 1D WAXD results and POM observations, the block copolymers with DP of PiPCS segment higher than 168 melted into isotropic liquid first upon heating and then formed a stable Col_h LC phase within the microphase separated structure at a temperature of about 160 °C and no clearing point was detected until decomposition.

Fig. 4 (a) Polarized optical microscopy microphotographs of PiBMA313–PiPCS168 taken at 170 °C. The polymer film, with thickness of about 10 microns, was annealed at 170 °C for at least 5 h before the picture was taken. Temperature-variable one-dimensional wide-angle X-ray diffraction powder patterns of PiBMA313–PiPCS168 recorded during the first heating (b) and the subsequent cooling (c) processes.

Microphase separated structures

The bulk morphologies of the diblock copolymers were investigated by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The results for the three representative samples (PiBMA313–PiPCS168, PiBMA313–PiPCS267, and PiBMA313–PiPCS594) are shown in Fig. 5, and in the ESI (Fig S1, and S2†). Their molecular weights and compositions are listed in Table 1.

Fig. 5 Sets of SAXS patterns of an unorientated PiBMA313–PiPCS168 sample recorded by using SAXSess at various temperature during the first heating processes in the low-q (a) region and high-q region (b); and a TEM image at 170 °C annealed 8 h.

The SAXS profiles provided a clear evidence for microphase separated structures for PiBMA313–PiPCS168, PiBMA313–PiPCS267, and PiBMA313–PiPCS594. Fig. 5a revealed the phase morphology of PiBMA313–PiPCS168 with the f^PMPCS of 50.4%, with only one peak at q = 0.19 nm⁻¹, which suggested that the block copolymers were microphase separated with a d-spacing of 33 nm. The absence of higher-order peaks might be due to the composition profile across the interface being rather smooth and/or no significant long-range order of the microphase separated structure established in the block copolymer. In Fig. 5b, a scattering halo at q of 4.78 nm⁻¹ attributed to the amorphous PiPCS block was observed below 170 °C during the first heating. When the temperature was kept constant at 170 °C, the scattering peak at q of 4.78 nm⁻¹ started to increase in intensity; and afterwards, a sharp diffraction at 4.61 nm⁻¹ (i.e., d spacing is 1.36 nm) rapidly developed. This peak, which corresponded to the strong diffraction peak in 1D WAXD described previously, had been assigned as the Col_h phase formed by the PiPCS blocks.^35,36 With the liquid crystalline phase of PiPCS blocks developing, changes occurred in the SAXS curve: the peak at q = 0.19 nm⁻¹ suddenly shifted toward lower q and the higher-order reflections appeared (q = 0.14, 0.29, and 0.43 nm⁻¹; d = 45, 22, and 15 nm). Evidently, the lamellar microphase separated structure developed, indicating that the coil-to-rod conformational change of PiPCS block induced an ordered microphase separated structure in this sample. The microstructures of PiBMA313–PiPCS168 at different temperatures were examined using TEM. From the TEM micrographs, alternating black and white layers were easily discernible in the sample obtained at 170 °C. The black layers are ascribed to the PiPCS domains, and the white to the PiBMA domains (Fig. 5c). In this case, the periodicity was about 43 nm, which was consistent with 45 nm obtained by SAXS.

At a higher PiPCS volume fraction of 61.8%, a lamellar-to-lamellar transition was observed. The sample rendered the first-order scattering peak at q of 0.17 nm⁻¹ which corresponded to a d-spacing of 37 nm in the SAXS patterns. The second-order scattering peak was also visible at q of 0.34 nm⁻¹ at room temperature. The ratio of the scattering vectors of the peaks was 1 : 2, indicating that a lamellar phase formed. Below 170 °C, this scattering peak only slightly shifted toward lower q upon heating. The substantial change of the phase morphology occurred at a temperature of 170 °C, wherein the first-order scattering peak suddenly moved to a lower q value and reached a peak position at 0.14 nm⁻¹ (d = 46 nm). Higher-order reflections were also visible at scattering vector positions of two and three times that of the low-q peak, indicating the occurrence of an order-order transition. The ordered structures were identified using TEM. Lamellar phase was observed at different temperatures and the lamellar phase tended to be more well-defined with increasing temperature. In this case, the periodicity changed from 37 nm to 45 nm, which was consistent with that of 40 nm to 50 nm observed by TEM. The detailed experiment results are shown in the ESI, Fig. S1.†

A PiPCS-rich diblock copolymer PiBMA313–PiPCS594 possessing an f^PMPCS of 78% was also designed to study the effect of conformational transition of PiPCS on BCP self-assembly. From SAXS patterns (ESI, Fig. S2†), hexagonal structure was observed at room temperature with a primary peak found to be centered at a scattering vector q of 0.13 nm⁻¹ (d = 47 nm). Higher-order reflections were also visible at scattering vector positions of √3, √4 and √9 times of the low-q peak, indicting a microstructure of hexagonally packed cylinders. When the sample was heated, higher-order reflections disappeared. With the PiPCS domain entering the LC phase at 160 °C, the primary peak shifted to a lower q value corresponding to a d-spacing of 55 nm, and there was a higher-order reflection at a scattering vector position of twice of that of the first peak, indicative of the formation of a lamellar structure. In the TEM micrograph, there were no regular hexagonal structures but a PiBMA columnar structure for the sample at room temperature which might be attributed to the hexagonal structures not being stable, and the adjustment of the structure when the sample was embedded into epoxy at 80 °C. The TEM micrograph gave a clear view of structural transition, and it was in agreement with the SAXS data. The detailed experiment results are shown in the ESI, Fig. S2.†

The phase behavior of AB block copolymers with two amorphous blocks is determined by χN, where χ is the Flory-Huggins parameter and N is the total degree of polymerization. The PiPCS block was a coil before entering the LC phase, and a lamellar morphology was expected for sample PiBMA313–PiPCS168, on the basis of its composition according to the phase diagram of the diblock copolymer with two amorphous blocks.⁴⁰ The morphology of PiBMA313–PiPCS168 was difficult to confirm because of the similar chemical structural units of iBMA and iPCS. With PiPCS content increasing with increasing N, a lamellar phase was obtained for PiBMA313–PiPCS267 and a PiBMA columnar phase for PiBMA313–PiPCS594. PiPCS would transform into Col_h LC phase and thereafter serve as a rod in the block copolymers studied. Coil-to-rod conformational change resulted in a substantial increase in χ, and packing of the rod and coil blocks at the interface in turn dominated the phase separation process. This leads to the evolution of the original microphase separated structures observed with coil PiPCS block into a lamellar morphology with a significantly increased long period. This microphase separated structure transition was triggered by the amorphous-to-LC transition and was irreversible. According to the results, the bulk morphologies of the diblock copolymers were influenced by the PiPCS content and the conformation of PiPCS (Chart 1).

Chart 1 Schematic representation of different hierarchical structures of the diblock copolymers.

Conclusions

We have synthesized a series of diblock copolymers with different molecular weights, compositions, and low molecular weight distributions. Hexagonal columnar liquid crystalline phase (Col_h) was generated in the PiPCS domains of the block copolymers lamellar microstructure with DP of PiPCS, which contained no traditional mesogenic units higher than 168. The block copolymers distinguished themselves from other liquid-crystalline/amorphous diblock copolymers by the fact that they first became isotropic liquid upon heating and then ordered into Col_h phase. The isotropic-to-LC transition (coil-to-rod transition of PiPCS) made the original microphase separated structure observed with the coil PiPCS block evolve into a lamellar morphology of a significantly increased long period.

Acknowledgements

The financial support of the National Natural Science Foundation of China (Grants 20674001 and 20834001) and the National Distinguished Young Scholar Fund (Grant 20325415) is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, X-ray scattering profiles and TEM pictures. See DOI: 10.1039/b915519h

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