Directed self-assembly of block copolymers on chemical patterns: A platform for nanofabrication

Directed self-assembly (DSA) of block copolymers (BCPs) on lithographically defined chemically nanopatterned surfaces (or chemical patterns) combines advantages of conventional photolithography and polymeric materials and shows promise for meeting a sufficiently inclusive set of manufacturing constraints for applications in semiconductors and data storage. DSA attracts attention from both academia and industry and tremendous progress has been achieved in the past decade. This review highlights the development of DSA with an emphasis on efforts toward the integration of block copolymer lithography into the current lithographic process for the fabrication of devices for integrated circuits and bit-patterned media.     

Nine-fold density multiplication of hcp lattice pattern by directed self-assembly of block copolymer

Block copolymer assembly directed by electron beam (EB) lithography enhances both resolution and throughput of the EB-generated patterns and provides a feasible path to fabricating master molds of nanometer scale patterns over macroscopic areas. In our previous paper [27], we demonstrated that the self-assembly process can interpolate points in between the EB-generated pattern, thus attaining four-fold density multiplication. Here, we report a nine-fold feature density multiplication can be attained by the directed block copolymer assembly. The equilibrium formation of perpendicular cylindrical domains in registration with the pre-patterned surface is confined within a narrow thickness range once all other parameters are fixed as found in a four-fold feature density multiplication. The tolerance of the lattice mismatch between chemical pattern and d spacing of domains for nine-fold feature density multiplication is smaller than that for four-fold feature density multiplication. We also found that the critical dimension formed by the block copolymer domains is independent of that defined by the EB pre-patterned features.     

偶氮嵌段共聚物合成、自组装与光响应性

含偶氮苯基团的嵌段共聚物兼具偶氮聚合物和嵌段共聚物的特点,可自组装形成具有特殊光响应性能的各种纳米/亚微米结构,在光信息存储、传感器、药物输送、光控智能材料等领域有重要的应用前景。本文对本课题组近年来在含强推拉电子型偶氮生色团的嵌段共聚物合成、自组装与光响应性方面的研究工作进行简要综述。     

Polystyrene-poly(2-ethylhexylmethacrylate) block copolymers: Synthesis,bulk phase behavior,and thin film structure

Anionic polymerization was employed to synthesize well-defined diblock copolymers of polystyrene and poly(2-ethylhexylmethacrylate), PS-PEHMA. Diblock morphologies in bulk and in substrate-supported thin films were characterized by small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM), respectively. PS-PEHMA diblocks exhibited thermotropic order-disorder transitions; one diblock showed a thermoreversible transition between lamellae and a higher-temperature morphology assigned as perforated lamellae. Unlike PS-poly(alkylmethacrylate) diblocks where the alkyl group is n-butyl or n-pentyl, PS-PEHMA diblocks showed a typical decreasing Flory interaction parameter with increasing temperature. Thin films of PS-cylinder-forming PS-PEHMA diblocks showed a strong preference for the cylinders to lie in the plane of the film; films of incommensurate thickness readily formed terraces. Films of commensurate thickness were easily aligned over macroscopic areas through the application of mechanical shear.     

丙烯腈嵌段共聚物的合成及其自组装研究进展

综述了含聚丙烯腈(PAN)嵌段共聚物的合成方法及其在溶液中的自组装技术。对常用的活性自由基聚合方法,如原子转移自由基聚合(ATRP)、可逆加成断裂链转移(RAFT)聚合、氮氧自由基聚合(NMP)以及钴调介自由基聚合(CMRP)等方面的研究进行了总结,同时对PAN类嵌段共聚物在溶液中的自组装技术进行了概括。最后提出了现有技术存在的问题,并对其今后发展方向进行了展望。     

Evolution of interphase in styrene-butadiene block copolymers as revealed by H solid-state NMR: Effect of temperature and molecular architecture

¹H spin-diffusion solid-state NMR, in combination with other techniques, was utilized to investigate the effect of molecular architecture and temperature on the interphase thickness and domain size in poly(styrene)-block-poly(butadiene) and poly(styrene)-block-poly(butadiene)-block-poly(styrene) copolymers (SB and SBS) over the temperature range from 25 to 80 °C. These two block copolymers contain equal PS weight fraction of 32 wt%, and especially, polystyrene (PS) and polybutadiene (PB) blocks are in glass and melt state, respectively, within the experimental temperature range. It was found that the domain sizes of the dispersed phase and interphase thicknesses in these two block copolymers increased with increasing temperature. Surprisingly we found that the interphase thicknesses in these two block copolymers were obviously different, which was inconsistent with the theoretical predictions about the evolution of interphase in block copolymer melts by self-consistent mean-field theory (SCFT). This implies that the interphase thickness not only depends strongly on the binary thermodynamic interaction (χ) between the PS and PB blocks, but also is influenced by their molecular architectures in the experimental temperature range.     

Comparison of thermomechanical properties of statistical, gradient and block copolymers of isobornyl acrylate and n-butyl acrylate with various acrylate homopolymers

Well-defined statistical, gradient and block copolymers consisting of isobornyl acrylate (IBA) and n-butyl acrylate (nBA) were synthesized via atom transfer radical polymerization (ATRP). To investigate structure-property correlation, copolymers were prepared with systematically varied molecular weights and compositions. Thermomechanical properties of synthesized materials were analyzed via differential scanning calorimetry (DSC), dynamic mechanical analyses (DMA) and small-angle X-ray scattering (SAXS). Glass transition temperature (T_g) of the resulting statistical poly(isobornyl acrylate-co-n-butyl acrylate) (P(IBA-co-nBA)) copolymers was tuned by changing the monomer feed. This way, it was possible to generate materials which can mimic thermal behavior of several homopolymers, such as poly(t-butyl acrylate) (PtBA), poly(methyl acrylate) (PMA), poly(ethyl acrylate) (PEA) and poly(n-propyl acrylate) (PPA). Although statistical copolymers had the same thermal properties as their homopolymer equivalents, DMA measurements revealed that they are much softer materials. While statistical copolymers showed a single T_g, block copolymers showed two T_gs and DSC thermogram for the gradient copolymer indicated a single, but very broad, glass transition. The mechanical properties of block and gradient copolymers were compared to the statistical copolymers with the same IBA/nBA composition.     

Hydrogenated linear block copolymers of butadiene and isoprene: effects of variation of composition and sequence architecture on properties

The effect of variations in molecular architecture and composition on bulk properties is reported for a series of well characterized hydrogenated block copolymers of butadiene (HB) and isoprene (HI), each with a total molecular weight of ～ 200 000 and a narrow distribution ($\text{Mw}\text{Mn}\text{<1.17}$). The polymers were synthesized by sequential anionic polymerization followed by hydrogenation, using p-toluenesulphonylhydrazide. The material properties of the homopolymeric HI and HB were also investigated. As expected, HI is rubbery at room temperature and HB is a tough semicrystalline plastic with properties similar to those of a low density polyethylene, LDPE. The crystallinity, density and ΔH_t for all of the block copolymers were found to be linearly dependent on HB content indicating that little mixing exists between the HB and HI blocks in the solid state. Although the solution cast films of the block copolymers were spherulitic, the quenched films displayed no distinct structure on the supermolecular level indicating that the aggregation of the crystallites was more random in these films. The stress-strain properties of triblock copolymers with different block sequence, HBIB and HIBI, and a di-block copolymer, HBI, were similar in bulk behaviour to each other in the high and the intermediate butadience content (50–90%). This was related to the fact that the mechanical properties were determined predominantly by the behaviour of the more continuous HB phase. For the lower butadiene compositions (7–29%), there was a major difference in the behaviour of polymers with different block architecture. HBIB polymers were thermoplastic elastomers, whereas HIBI polymers behaved like an uncured particulate filled rubber. This difference was related to the presence of permanent ‘entanglements’ in HBIB polymers. The permanent entanglements which act as a physical crosslink are a consequence of the anchorage of the HB end blocks in the semicrystalline domains. No such arrangement is possible for either the HIBI or HBI polymers. The hysteresis behaviour of HBIB polymers were strongly dependent on butadiene content, decreasing with lowering of the concentration of the semicrystalline HB. This dependence was related to the continuity of the crystalline microdomains. All the members of HIBI series (and the HBI we considered) showed large hysteresis behaviour. This large energy loss during cyclic deforn ation in these polymers was related to the absence of the permanent anchor points arising from end block crystallization.     

Diblock copolymers with an amorphous, high glass transition temperature, organometallic block: synthesis, characterisation and self-assembly of polystyrene-b-poly(ferrocenylisopropylmethylsilane) in the bulk state

Living anionic ring-opening polymerisation of isopropylmethylsila[1]ferrocenophane yields poly(ferrocenylisopropylmethylsilane) (PFiPMS) with controlled molecular weights and narrow polydispersities up to M_n = ca. 20,000 Da. Polystyrene-b-poly(ferrocenylisopropylmethylsilane) (PS-b-PFiPMS) diblock copolymers have been prepared via sequential living anionic polymerisation. These materials are examples of diblock copolymers with an amorphous, organometallic block with a glass transition temperature (T_g) above room temperature (60 °C). High molecular weight diblock copolymers (M_n = 42,000–51,000 Da) were targeted with low polydispersities (PDI = 1.1). As both blocks are amorphous, these materials self-assemble into predictable morphologies in the bulk state with well-ordered nanodomains.     

Design of novel poly(methyl vinyl ether) containing AB and ABC block copolymers by the dual initiator strategy

A new set of block copolymers containing poly(methyl vinyl ether) (PMVE) on one hand and poly(tert-butyl acrylate), poly(acrylic acid), poly(methyl acrylate) or polystyrene on the other hand, have been prepared by the use of a novel dual initiator 2-bromo-(3,3-diethoxy-propyl)-2-methylpropanoate. The dual initiator has been applied in a sequential process to prepare well-defined block copolymers of poly(methyl vinyl ether) (PMVE) and hydrolizable poly(tert-butyl acrylate) (PtBA), poly(methyl acrylate) (PMA) or polystyrene (PS) by living cationic polymerization and atom transfer radical polymerization (ATRP), respectively. In a first step, the Br and acetal end groups of the dual initiator have been used to generate well-defined homopolymers by ATRP (resulting in polymers with remaining acetal function) and living cationic polymerization (PMVE with pendant Br end group), respectively. In a second step, those acetal functionalized polymers and PMVE-Br homopolymers have been used as macroinitiators for the preparation of PMVE-containing block copolymers. After hydrolysis of the tert-butyl groups in the PMVE-b-ptBA block copolymer, PMVE-b-poly(acrylic acid) (PMVE-b-PAA) is obtained. Chain extension of the AB diblock copolymers by ATRP gives rise to ABC triblock copolymers. The polymers have been characterized by MALDI-TOF, GPC and ¹H NMR.     

Synthesis of novel random and block copolymers of tert-butyldimethylsilyl methacrylate and methyl methacrylate by RAFT polymerization

Hydrophobic-hydrolysable copolymers consisting of methyl methacrylate (MMA) and tert-butyldimethylsilyl methacrylate (TBDMSMA) have been synthesized for the first time by Reversible Addition-Fragmentation chain Transfer (RAFT) polymerization technique using cumyl dithiobenzoate (CDB) and cyanoisopropyl dithiobenzoate (CPDB) as chain transfer agents (CTAs). The monomer reactivity ratios for TBDMSMA (r₁ = 1.40 ± 0.03) and MMA (r₂ = 1.08 ± 0.03) have been determined using a non-linear least-squares fitting method. Well-defined random copolymers PMMA-co-PTBDMSMA have been prepared. Then, the versatility of the RAFT process to synthesize silylated block copolymers with controlled molecular weights and low polydispersities has been demonstrated using two strategies: the synthesis of PMMA-SC(S)Ph or PTBDMSMA-SC(S)Ph as macro-chain transfer agent (macro-CTA) for use in a two step method or an one-pot method which consists in the successive addition of the two monomers. Diblock copolymers with narrow molecular weight distributions (PDI < 1.2) were obtained from the one-pot method with number-average molecular weight values within the range 10,000-22,000 g mol⁻¹.     

含聚乙二醇的嵌段共聚物的合成及自组装研究进展

综述了通过活性自由基聚合如原子转移自由基聚合（ATRP）、氮氧稳定自由基聚合（NMP）、可逆加成断裂链转移聚合（RAFT）等方法合成含聚乙二醇（PEG）的嵌段共聚物的研究进展,并对含PEG类嵌段共聚物在溶液中的自组装技术和在药物载体、介孔材料以及碳纳米管中的应用进行了归纳,指出含PEG的嵌段共聚物可以自组装成多种形貌,直接影响材料的性能和应用,所以这些结构有潜在的应用价值和应用前景,并且合成新的含PEG的嵌段共聚物和开发具有新型结构、形貌可控的自组装体以及新的应用领域是今后的一个热点问题,具有重要的科学研究意义和实际应用价值。     

The amphiphilic block copolymers of 2-(dimethylamino)ethyl methacrylate and methyl methacrylate: Synthesis by atom transfer radical polymerization and solution properties

Amphiphilic di- and tri-block copolymers of poly(methyl methacrylate) (PMMA) and poly(2-dimethylamino)ethyl methacrylate (PDMAEMA) have been synthesized by atom transfer radical polymerization (ATRP) at ambient temperature (35 °C) in the environment-friendly solvent, aqueous ethanol (water 16 vol%) using CuCl/o-phenanthroline as the catalyst. The PDMAEMA blocks are contaminated with ethyl methacrylate (EMA) residues to the extent of 1-2 mol% of DMAEMA depending on the length of the PDMAEMA block. The EMA forms through the autocatalyzed ethanolysis of the DMAEMA monomer and undergoes random copolymerization with the latter. The rate of ethanolysis is unexpectedly greater in the aqueous ethanol than in neat ethanol, which has been attributed to the higher polarity of the former than of the latter. In contrast to the ethanolysis no hydrolysis of DMAEMA in the aqueous ethanol medium could be detected for 133 h. The block copolymers form micelles in water. Their solubility and CMC in neutral water have been studied. Dynamic light scattering (DLS) studies reveal that for a fixed degree of polymerization (DP) of the PMMA block the hydrodynamic diameter of the micelles in methanolic water (water 95 vol%) increases at a faster rate with the DP of the PDMAEMA block when it is much greater than that of the PMMA block compared to when it is less than or close to that of the latter.     

Synthesis and characterization of side-chain liquid crystalline homopolymers and block copolymers containing biphenyl-4-ylthiophene and biphenyl-4-ylfluorene pendants

A series of new liquid crystalline homopolymers (P1 and P2) and block copolymers (P3 and P4) composed of methacrylates containing pendant biphenyl-4-ylthiophene (M1) and biphenyl-4-ylfluorene (M2) units were synthesized by atom transfer radical polymerization (ATRP). The number-average molecular weights (M_n) of the homopolymer (P2) and diblock copolymers (P3 and P4) were in the range of 5153-8713 g mol⁻¹ with polydispersity indices (PDIs) between 1.17 and 1.25. The thermal, mesogenic, and photoluminescence (PL) properties of all polymers were investigated. Except for the absence of mesogenic properties in block copolymer P4, polymers P1 and P3 possessed the smectic A phase and polymer P2 exhibited the nematic phase. Moreover, the mesomorphism and the layer d-spacing values of the smectic A phase in polymers P1 and P3 were confirmed and characterized by X-ray diffraction (XRD) patterns.     

Synthesis of thermoresponsive block and graft copolymers via the combination of living cationic polymerization and RAFT polymerization using a vinyl ether-type RAFT agent

A novel vinyl ether-type RAFT agent, benzyl 2-(vinyloxy)ethyl carbonotrithioate (BVCT) was synthesized for various block copolymers via the combination of living cationic polymerization of vinyl ethers and reversible addition−fragmentation chain transfer (RAFT) polymerization. The novel BVCT–trifluoroacetic acid adduct play an important role to produce well-defined block copolymers, which is both as a cationogen under EtAlCl₂ initiation system in the presence of ethyl acetate for living cationic polymerization and a RAFT agent for blocks by RAFT polymerization. The resulting polymer, poly(vinyl ether)s, by living cationic polymerization had a high number average α-end functionality (≥0.9) as determined by both ¹H NMR and MALDI-TOF-MS spectrometry. In addition, this poly(vinyl ether)s worked well as a macromolecular chain transfer agent for RAFT polymerization. The RAFT polymerization of radically polymerizable monomers was conducted in toluene using 2,2′-azobis(isobutyronitrile) at 70 °C. For example, a double thermoresponsive block copolymer (MOVE₆₁-b-NIPAM₁₅₀) consisting of 2-methoxyethyl vinyl ether (MOVE) and N-isopropylacrylamide (NIPAM) was prepared via the combination of living cationic polymerization and RAFT polymerization. The block copolymer reversibly formed and deformed micellar assemblies above the phase separation temperature (T_ps) of poly(NIPAM) block in water. This BVCT is not only functioned as an initiator, but also acted as a monomer. When BVCT was copolymerized with MOVE by living cationic polymerization, followed by graft copolymerization with NIPAM via RAFT polymerization, well-defined graft copolymers (MOVE_n-co-BVCT_m)-g-NIPAM_x (n = 62–73, m = 1–9, x = 19–214) were successfully obtained. However, no micelle formed in water above T_ps of poly(NIPAM) graft chain unlike the case of block copolymers.     

Structure of insoluble complex formed by a block copolymer of 2-ethyl-2-oxazoline and ethylene oxide and poly(methacrylic acid)

Complexes that were insoluble in water were formed by mixing aqueous solutions of a block copolymer of 2-ethtyl-2-oxazoline (EOX) and ethylene oxide (EO) and those of poly(methacrylic acid) (PMAA). The structures of these complexes were investigated by the results obtained mainly by infrared spectroscopy, X-ray diffraction, and differential scanning calorimetry. The molar ratio of MAA in the complexes was also estimated by analyzing the infrared spectra. Whereas homopolymers of EOX and EO formed nearly equimolar complexes with PMAA irrespective of the feed molar ratio, the molar ratio of MAA in the complexes formed by the block copolymer and PMAA depended on the feed molar ratio. Although the infrared spectra indicated structural differences between the homopolymer of EOX and EOX in the block copolymer before forming complexes, the spectra obtained for the complexes formed by the homopolymer and the block copolymer were similar to each other.     

Synthesis and properties of ABA-type triblock copolymers of poly(glycolide-co-caprolactone) (A) and poly(ethylene glycol) (B)

Poly(glycolide-co-caprolactone) (A)-poly(ethylene glycol) (B) ABA-type triblock copolymers (PGCE) were synthesized by bulk ring opening polymerization, using the hydroxyl endgroups of poly(ethylene glycol) (PEG) as initiator and stannous octoate as catalyst. The resulting copolymers were characterized by various analytical techniques. Gel permeation chromatographic analysis indicated that the polymerization product was free of residual monomers, PEG and oligomers. ¹H NMR and differential scanning calorimeter results demonstrated that the copolymers had a structure of poly(glycolide-co-caprolactone) (PGC) chains chemically attached to PEG segments. All the PGCE copolymers showed improved hydrophilicity in comparison with the corresponding PGC copolymers with the same molar ratio of glycolidyl and caproyl units. The microspheres of PGCE copolymer exhibited rough surfaces quite different from the smooth surface of PGC microspheres. This phenomenon was attentively ascribed to the highly swollen ability of PGCE copolymers and the freeze-drying process in the microspheres fabrication.     

Mesoscale simulation of block copolymers in aqueous solution: parameterisation, micelle growth kinetics and the effect of temperature and concentration morphology

In this work, we utilise ‘MesoDyn’ [J Chem Phys 99 (1993) 9202; 106 (1997) 4260] density functional simulations to study the effect of temperature and concentration on the micellar morphology of polymeric surfactants. Parameterisation strategies based upon atomistic models and experimental data are discussed. Taking the temperature dependence of interaction energy into account, the change in morphology of Pluronic (PEO-PPO-PEO) block copolymer structure with temperature is well reproduced. As a function of concentration, the diameter of spherical micelles is found to increase in line with previous cryo-TEM observations [Phys Chem Chem Phys 1 (1999) 3331]. Simulations of high concentration PEO-PBO diblock systems show ordering similar to the face-centered cubic structures found experimentally [J Polym Sci B 33 (1995) 1085; Macromolecules 30 (1997) 5721; Polymer 39 (1998) 4891; Phys Chem Chem Phys 1 (1999) 2773].     

SPTES-b-PI质子交换膜的制备及表征

质子交换膜作为质子交换膜燃料电池的核心部件具有提供离子通道传递质子和隔绝两极气体的双重作用,其性能的好坏直接影响着电池性能的优劣。主链引入亲水和疏水段的嵌段芳香族共聚物,由于各嵌段之间具有热力学不相容性会产生微相分离结构,进而形成高效的质子传导通道。本文以磺化双（4-氟苯基）砜（SDFDPS）和4,4'-硫代双苯硫酚（TBBT）为单体,以间羟基苯胺为封端剂合成了带有氨端基的磺化聚芳硫醚砜（SPTES-NH₂）。嵌段聚合物SPTES-b-PI通过亲水段SPTES-NH₂与以1,4,5,8-萘四羧酸二酐（NDA）和4,4'-双（3-氨基苯氧基）二苯基砜（m-BAPS）为单体缩聚而成的疏水段聚酰亚胺（PI）的酰亚胺化偶联反应来合成,制备出了PI分子量不同的SPTES-b-PIx（x=5~20kg/mol）。SPTES-b-PIx膜显示出优异的热力学稳定性,SPTES-b-PIx膜的脱磺化反应开始于290℃高于260℃的SPTES膜,与SPTES-70相比吸水率降低。随着聚酰亚胺分子量的增大,热稳定性增加,质子传导率增加。SPTES-b-PIx的质子传导率25℃下达到0.045~0.124S/cm。     

Synthesis,properties and degradation of polyisobutylene–polyester graft copolymers

The development of copolymers is a promising approach for combining the favorable properties of two polymers and obtaining new properties of the combination. In this work, graft copolymers of polyisobutylene (PIB ) and polycaprolactone or poly(d ,l ‐lactide) were synthesized and studied. Amine‐terminated polyesters were synthesized and were grafted onto an activated PIB backbone synthesized from butyl rubber, a copolymer of isobutylene and 2 mol% isoprene. The polyester content was tuned from 15 to 44 wt% by varying the molar mass of the polyester blocks and the number of molar equivalents used in the grafting reaction. The graft copolymers with higher polyester content underwent nanoscale phase separation, as demonstrated by differential scanning calorimetry and atomic force microscopy imaging. This was found to provide enhanced mechanical properties such as increased tensile strength and Young's modulus relative to the starting rubber or physical blends. Despite the significant polyester content of the graft copolymers and the susceptibility of the polyesters to degradation, the graft copolymers underwent negligible mass loss in 5 mol L^?1 NaOH over a period of eight weeks. These results suggest that polyesters can be incorporated into PIB to tune and enhance its properties, while maintaining high chemical stability. © 2016 Society of Chemical Industry