(265a) Microphase Separation in Hydrogen-Bonding Block-Random Copolymers of Styrene and 4-Acetoxystyrene

The strength and nature of interactions within a block copolymer will dictate its phase behavior. Nonspecific dispersion forces are weaker than specific hydrogen bonding forces, so introducing low levels of hydrogen boding can lead to microphase separation in a polymer which would otherwise be disordered. Chemical modification of a block copolymer allows introduction of hydrogen-bonding functional groups and provides a tool for the study of the transition from the homogeneous to microphase-separated regime. Block copolymers and block-random copolymers of styrene and 4-acetoxystyrene can be synthesized via nitroxide-mediated radical polymerization^1,2. This controlled radical polymerization method is suited to this copolymer system due to the tolerance of radicals to the functional group in 4-acetoxystyrene and the ability to form a statistical copolymer block within the block copolymer³. The ester group in poly(4-acetoxystyrene) (PAS) can be cleaved to form a phenol, yielding poly(4-hydroxystyrene) (PHS). Control over the concentration of hydrogen bonding groups is obtained by varying the extent of this cleavage reaction. In a suitable block-random copolymer, the all-acetoxy form will form a homogeneous melt, while the phenol form will show a microphase-separated melt due to the hydrogen bonding interactions. Partial ester cleavage provides a range of copolymers to observe the transition between these two limiting cases. Small angle X-ray scattering is used to observe the nanoscale structure in these copolymers, providing insight into how the introduction of specific interactions drives phase separation in block copolymers.

Experimental

TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl,98%), benzoyl peroxide (97%), 4-acetoxystyrene (96%), (1R)-(-)-10-camphorsulfonic acid (98%), and hydrazine solution (1.0 M in tetrahydrofuran) were used as received from Aldrich. Styrene (99%, Aldrich) was vacuum distilled from dibutylmagnesium. The alkoxyamine initiator, 1-benzyloxy- 2-phenyl- 2- (2',2',6',6'-tetramethyl-1-piperidinyloxy) ethane, was synthesized based on the procedure of Hawker⁴. Benzoyl peroxide (5.041 g) was added to a TEMPO (2.718 g) solution in styrene (60 mL), and the mixture was degassed by freeze-pump-thaw. The mixture was allowed to stir at 60°C for 19 hr, then cooled to 0°C. The solids were removed by filtration, and unreacted styrene was removed by vacuum distillation. The product was isolated by column chromatography (24:1 hexane : ethyl acetate, silica gel), and confirmed by ¹H NMR.

Styrene was polymerized from the alkoxyamine initiator in mixed xylenes at 120°C for 18-48 hr to obtain the polystyrene macroinitiator. The polymer was isolated by precipitation into methanol. Molecular weight was controlled by reaction time and monomer-to-initiator ratio. PS-b-PAS was synthesized using 4-acetoxystyrene as the monomer with the polystyrene macroinitiator and camphorsulfonic acid for rate enhancement in xylenes at 120°C. PS-b-(PS-stat-PAS) was synthesized from a mixture of styrene and 4-acetoxystyrene with the polystyrene macroinitiator and with camphorsulfonic acid in xylenes at 120°C. Crude copolymer products were isolated by precipitation into methanol. Copolymers were freed from PS homopolymer by precipitation into cyclohexane. Product functional group composition was determined by ¹H NMR.

The PS-PAS copolymer was dissolved in tetrahydrofuran. Hydrazine solution in THF was added in slight excess for complete conversion, or in an appropriate amount for partial conversion. Reaction was allowed to proceed for 18 hr, then a slight excess of HCl was added to stop the reaction. The product was precipitated into DI water, filtered, and washed with DI water. Conversion was checked with ¹H NMR.

Gel permeation chromatography in THF was used for determination of molecular weight. The system consists of two 30 cm Polymer Laboratories PLgel Mixed-C columns, Waters 515 HPLC Pump, Waters 410 Differential Refractometer, Waters 2487 Dual λ Absorbance Detector, and Precision Detectors PD2020 Light Scattering System. ¹H NMR spectra were acquired on a 300 MHz Varian Mercury-VX. X-Ray scattering patterns were collected on a system consisting of a Philips PW3830 X-Ray generator with CuKα radiation, an Anton Paar compact Kratky camera with a custom hotstage, and an MBraun OED-50M position sensitive detector.

Results and Discussion

Nitroxide-mediated radical polymerization (NMRP) can be used in the synthesis of block copolymers of styrene and 4-acetoxystyrene. Styrene and many substituted styrenes propagate in a controlled fashion with NMRP and sequential polymerizations with different monomers will result in block copolymer formation. Since propagation is by free radical chemistry, the polymerization is tolerant of the functional group in 4-acetoxystyrene, unlike in anionic polymerization. This chemistry also allows for the synthesis of copolymers with a block-random architecture as the radical reactivity ratios allow the formation of random copolymers, unlike ionic polymerization chemistries. In the case of the styrene/4-acetoxystyrene system the reactivity ratios are close to unity (r1=0.88, r2=1.18)⁵, so composition drift in the system will be minimal if high conversions are avoided. This allows for the formation of a block-random copolymer with little composition gradient within the random copolymer block.

Block copolymers and block-random copolymers are synthesized by first polymerizing styrene with an alkoxyamine initiator to form a PS macroinitiator. This first step yields a narrow-distribution polymer with a controlled molecular weight and an alkoxyamine end group. The second block is synthesized from this PS macroinitiator, with either 4-acetoxystyrene or a mixture of 4-acetoxystyrene and styrene as the monomer. GPC shows a shift to higher molecular weight as the additional block grows on the end of the preexisting PS block. A low molecular weight shoulder exists consisting of PS homopolymer that did not initiate growth of a second block, but the copolymer can be isolated by precipitation into cyclohexane, which is a marginal solvent for PS but a nonsolvent for PAS. These PS-PAS copolymers are then modified by cleavage of the ester group in PAS to a phenol. NMR shows the disappearance of the methyl protons in the acetoxy group, showing that the resulting polymer is now a PS-PHS copolymer.

PS-PAS copolymers have been investigated by small angle X-ray scattering (SAXS). In two different block copolymers with a second block composed of 100% PAS, sharp primary peaks were observed , as well as weaker peaks at twice the primary spacing, indicating a lamellar morphology for both near-symmetric polymers. The lamellar spacing d (=2π/q*, where q* is the primary peak position) is calculated as 42.5 and 17.5 nm for M_n of 43000 g/mol and 14000 g/mol, respectively. The lower molecular weight polymer provides a lower bound to the Flory interaction parameter for the polystyrene/poly(4-acetoxystyrene) system at χ = 0.1. In block-random copolymers, the interactions between the PS block and the PAS-containing block are weaker since the latter block is now diluted by styrene units. No peak is observed by SAXS for a symmetric copolymer with 50% by weight PAS in one block, even though the blocks are of similar molecular weight to a block copolymer that is phase segregated. This shows a homogeneous structure due to only a weak repulsion between the blocks. When the interactions become stronger, by cleaving a portion of the acetoxy units to phenol, phase segregation occurs and is observed with SAXS by the presence of a peak. This copolymer system has the desired qualities for studying how introducing functional groups influences phase behavior. Since the cleavage reaction can be carried out to a variable and controlled extent, the entire range of compositions can be investigated, with the transition from a homogeneous system expected to occur at a critical fraction of phenol groups. Results of these studies will be presented at the meeting.

Block copolymers and block-random copolymers of polystyrene and poly(4-acetoxystyrene) have been synthesized by nitroxide-mediated radical polymerization. This polymerization chemistry gives control over the molecular weight and composition of each block within the copolymer. Cleavage of the ester groups in poly(4-acetoxystyrene) to phenol groups results in poly(4-hydroxystyrene) and allows for the controlled introduction of hydrogen bonding groups into the polymer. Small angle X-ray scattering shows phase segregation in our PS-PAS block copolymers, but a homogeneous melt in a similar block-random copolymer, which has reduced interblock interactions. The same block-random copolymer with phenol groups is phase segregated. Varying specific interactions from the hydrogen bonding by phenol groups will change this interblock interaction, allowing study of phase behavior in this copolymer system.

Acknowledgements

This work is supported by the National Science Foundation through the Princeton Center for Complex Materials (DMR-0213706).

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