Article pubs.acs.org/Macromolecules
Controlling Interlamellar Spacing in Periodically Grafted Amphiphilic Copolymers Sananda Chanda and S. Ramakrishnan* Department of Inorganic and Physical Chemistry Indian Institute of Science, Bangalore 560012, India S Supporting Information *
ABSTRACT: We have examined a series of periodically grafted amphiphilic copolymers (PGACs) wherein pendant MPEG segments of varying lengths (TREG, 350, 550, and 750) were grafted at periodic intervals along a long crystallizable alkylene (C-20) backbone. The immiscibility of the alkylene and PEG segments, and the strong propensity of the alkylene segments to crystallize, drive these PGACs to selfsegregate by folding in a zigzag fashion and subsequently organize into a lamellar morphology, which was evident from DSC, SAXS, WAXS, and AFM studies; the interlamellar spacing was seen to increase linearly with MPEG length. Co-grafted PGACs, prepared by grafting a mixture of two different MPEGs (MPEG 350 and MPEG 750), also exhibited a lamellar morphology; interestingly, the interlamellar spacing was seen to depend only on the total PEG content, while the presence of a mixture of PEG lengths exerted little influence. However, a mixture of two homo-PGACs bearing either pendant MPEG350 or MPEG750 segments underwent macrophase separation, and interlamellar spacings corresponding to each were observed in the SAXS profiles. This study provides a new design for controlling the dimensions of the microphase-separated nanostructures at significantly smaller length scales (sub-10 nm) than is typically possible using block copolymers.
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INTRODUCTION Polymers carrying two or more immiscible segments exhibit different kinds of microphase-separated morphologies depending on the relative volume fractions of the segments. The majority of the studies to understand the evolution of the microphase morphology have been done using block copolymers;1 however, similar studies in graft copolymers have been more limited, although several studies have examined comb2 and comb-block copolymers.3 In block copolymers, the morphology depends not only on the strength of the immiscibility (denoted by χN, where χ is the interaction parameter and N is the total degree of polymerization) but also on whether the blocks are crystalline or amorphous; consequently, the Tg of the amorphous block(s), the crystallization temperature (Tc) of the crystalline block(s), and the order−disorder transition temperature (TOD) play crucial roles.4 Several reviews have described the interplay between the various factors that dictate the final morphology of the block copolymers.5 Because of their propensity to selfassemble at the molecular level, block copolymers exhibit a rich variety of spatially periodic nanostructures, and numerous efforts have been made to fine-tune the period of nanostructures;6 ultrasmall feature sizes could be of importance for a variety of applications.7 In the case of diblock and triblock copolymers, one of the basic methods to tune lamellar domain spacing is by varying the lengths of the blocks, but this often entails synthetic difficulties.8 An alternative way to control interlamellar spacing in block copolymer systems is by blending with homopolymers that are compositionally similar to one of © XXXX American Chemical Society
the blocks; however, the ability to tune the lamellar periodicity is sometimes hindered due to undesirable macrophase separation.9 Studies have also examined the swelling and shrinkage of lamellar domain of block copolymers on introduction of small molecular additives, such as a metal salt where control of the domain spacing is achieved by coordination between the metal atom and a functional group present in one of the blocks.10 Several studies have also examined the more subtle effects of dispersity in molecular weight of one of the blocks on the interlamellar spacing of diblock copolymers;11 it was seen that higher dispersity typically leads to an increase in the interlamellar spacing. Some years ago we designed polymers wherein the chain conformation is controlled by immiscibility between different segments;12 specifically, we prepared periodically grafted copolymers (PGAC) wherein the grafted segment is immiscible with the backbone. We showed that in such systems the polymer chain adopts a conformation so as to spatially segregate the backbone and graft segments; when the backbone segment has a strong propensity to crystallize, such as when it is a long alkylene unit, the chain adopts a zigzag folded conformation that permits the backbone to crystallize and the grafted segments get pushed to either sides of the folded chain, as shown in Scheme 1. Remarkably, independent crystallization of both the backbone and pendant segments was observed by Received: January 24, 2016 Revised: April 12, 2016
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DOI: 10.1021/acs.macromol.6b00162 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
and used directly. Eicosane-1,20-dioic acid was purchased from Tokyo Chemical Industry Co. Ltd. (TCI, Japan) and used without further purification. Dibutyl itaconate, 1,20-eicosanediol, and MPEG-thiols were prepared using procedures described earlier.15 Solvents used for synthesis were distilled prior to use and, if necessary, were dried following the standard procedures.16 Methods. 1H NMR spectra were recorded using a Bruker AV 400 MHz spectrometer in suitable deuterated solvents using tetramethylsilane (TMS) as internal reference. GPC studies were carried out using Waters GPC system with RI detector using THF as the eluent. Molecular weights were calculated using standard calibration curve based on the data from UV-PDA detector using narrow polystyrene standards. Thermal characterization was carried out using a PerkinElmer DSC instrument at a heating rate of 10 °C/min under a dry nitrogen atmosphere. Typically, 4−5 mg of the sample was taken, and two heating (excluding the first heating) and cooling runs were performed to ensure reproducibility. Small-angle X-ray scattering studies were performed using a Hecus S3-Micro System equipped with a one-dimensional position-sensitive detector. Data were recorded at a sample-to-detector distance 25.75 cm, using a wavelength, λ, of 1.542 Å (Cu Kα). Samples were taken in sealed glass capillaries, and the diffraction patterns were collected at 30 °C. The exposure time was set to 2000 s, and the diffraction patterns were calibrated using silver behenate. Wide-angle X-ray diffraction (WAXD) measurements were performed on a PANalytical Empyrean operating at 40 kV and 30 mA, using Cu Kα radiation (λ = 1.5418 Å) equipped with a PIXcel 3D detector. AFM measurements were performed using MultiMode SPM (Digital Instruments, Santa Barbara, CA) equipped with a NanoScope IV A controller. All the images presented are tapping mode height images, recorded using FESP (Veeco) tips of force constant 1−5 N/m and resonance frequency of 67 kHz. Analysis of the recorded images was done using the software provided with the instrument. General Polymerization Procedure: Poly(icosyl itaconate). 1 g (4.13 mmol) of dibutyl itaconate and 1.30 g (4.13 mmol) of 1,20icosanediol were taken in a test-tube-shaped vessel, and 52 mg (0.083 mmol) of dibutyltin dilaurate was added to it. 1.14 mg (0.010 mmol) of quinol was added as a radical quencher to prevent cross-linking.17 The polymerization was done under dry nitrogen purge for 12 h at 160 °C and then in a Kugelrohr at 165 °C under reduced pressure (
