Synthesis and solution properties of a temperature-responsive PNIPAM–b-PDMS–b-PNIPAM triblock copolymer

In this paper, we report the synthesis and self-assembly of a novel thermoresponsive PNIPAM60–b-PDMS70–b-PNIPAM60 triblock copolymer in aqueous solution. The copolymer used a commercially available precursor modified with an atom transfer radical polymerization (ATRP) initiator to produce an ABA triblock copolymer via ATRP. Small-angle neutron scattering (SANS) was used to shed light on the structures of nanoparticles formed in aqueous solutions of this copolymer at two temperatures, 25 and 40 °C. The poly(dimethylsiloxane) block is very hydrophobic and poly(N-isopropylacrylamide) (PNIPAM) is thermoresponsive. SANS data at 25 °C indicates that the solutions of PNIPAM–b-PDMS–b-PNIPAM copolymers form well-defined aggregates with presumably core–shell structures below cloud point temperature. The scattering curves originating from nanoparticles formed at 40 °C in 100% D2O or 100% H2O were successfully fitted with the Beaucage model describing aggregates with hierarchical structure.


Introduction.
Copolymers that combine blocks with different properties and different sensitivities to external stimuli in one structure attract great attention in soft matter research due to their potential biomedical applications. 1,2,3They also have application in the field of nanoarchitectronics. 4 New chemistry approaches have been tried along with physicochemical investigations of self-assembled structures including various types of micelles and vesicles, etc., formed by such copolymers.Small-angle X-ray and neutron scattering techniques can be used to take a "closer look" at the internal structures of nanoparticles.SAXS/SANS studies have been published on a variety of block copolymers such as the diblock copolymer poly(methoxy diethylene glycol acrylate)-block-polystyrene (PS), 5 the diblock copolymer PS-PNIPAM 6 , deuterated polystyrene and poly(n-hexyl methacrylate) (PnHMA), 7 poly(2-isopropyl-2-oxazoline)-b-poly(2-ethyl-2-oxazoline), 8 C18EO100 9 , polyethylene oxide -poly(2-vinylpyridine) 10 , polyethylene oxide -PNIPAM, 11,12 and PNIPAMpoly(n-butyl acrylate) 13,14 and on triblock copolymers such as (LCP, poly(4-cyanobiphenyl-4-oxyundecylacrylate)) 'A' endblock and a deuterated polystyrene 'B' midblock, 15 PS-PMDEGA-PS, 16,17 and PS-PNIPAM-PS 18 .The most studied class of temperature-responsive polymers are poly(ethylene oxide-block-propylene oxide-blockethylene oxide) PEO-PPO-PEO triblock copolymers known as Pluronics ® . 7Detailed SAXS/SANS studies of nanoparticle structures formed in aqueous solution have been published for a variety of commercially available Pluronics ® 19 such as L44, 20 L64, 21 F127, 22,23,24 P84, 25 P85, 26,27 P104, 20 L62, 28 L64, 29 L81, 16 F68, 30 F87, 16 and F88. 16 our previous work, the internal structure of PNIPAM-b-PEG-b-PNIPAM nanoparticles formed in aqueous solutions was inspected by SANS upon increasing temperature. 31polymers with deuterated (d-PEG) and hydrogenated central blocks (h-PEG) were synthesized to perform contrast variation experiments.Contrast variation experiments using SANS showed that the PNIPAM-b-PEG-b-PNIPAM copolymers below the cloud point existed as single polymer chains in a good solvent; a small portion of aggregates was also present in solution.In contrast, at higher temperatures, nanoparticles formed from PNIPAMb-PEG-b-PNIPAM copolymers had a non-uniform structure with "frozen" areas interconnected by single chains in a Gaussian conformation.Such "frozen" areas were attributed to PNIPAM domains interconnected with central PEG blocks that are uniformly distributed inside of a nanoparticle.
In this article, we report a new copolymer with a central poly(dimethylsiloxane) (PDMS) block, which is considerably more hydrophobic than PEG.The substitution of hydrophobic PDMS for hydrophilic PEG may have several consequences.We can expect a significant shift of CPT to much lower values.Another shift that may be foreseen is a change in chain conformation.Unimolecular micelles or compacted macromolecular chains might occur in solution if PEG is substituted by PDMS.
The main goal of this paper was to investigate self-assembly behaviour of novel thermoresponsive triblock copolymer by dynamic light scattering and SANS and to compare this knowledge with that obtained about PNIPAM-b-PEG-b-PNIPAM in our previous study.

Synthesis of the PDMS macroinitiator
PDMSdihydroxy (1.1 mM, 3.0 mL) and triethylamine (0.15 mL) were added to a dry, sealed, round-bottom flask containing THF (5.0 mL), with stirring.The solution was degassed by nitrogen bubbling for 20 min and then cooled to 0 °C in an ice-salt bath.BIBB (1.1 mMol, 0.13 mL) was then added dropwise, and the reaction was allowed to proceed overnight.The mixture was then filtered to remove triethylamine salts and filtered.The retentate was then washed with THF (2 x 25 mL), and all THF fractions were dried in vacuo to yield the PDMS macroinitiator (98% yield). 1 H NMR (400 MHz, CDCl3, δ): 1.85 (s, CH3), 0.00 (bs, Si-CH3) ppm.

Synthesis of PNIPAM-b-PDMS-b-PNIPAM
Copper(I) chloride (0.04 mMol, 4 mg) was added to a dried round-bottom flask and sealed in.
The flask was then degassed with nitrogen for 15 min.NIPAM (5.3 mMol, 600 mg), ME6TREN (0.04 mMol, 10.7 μL), and PDMS macroinitiator (0.04 mMol, 100 mg) were added to a separate dry flask, followed by THF (5 mL).The sealed THF solution was then degassed by bubbling with nitrogen for 20 min.Using a degassed syringe, the THF solution was transferred to the flask containing copper (I) chloride.The reaction was then allowed to proceed overnight at room temperature.The product was then passed through neutral alumina to remove copper from the reaction.The resulting solution was dried in vacuo and then dissolved in deionised water and extensively dialysed (3.5-5 kDa MWCO membrane, Visking) against water.Yield: 55 %.S1).
The copolymer was synthesised with a central PDMS block of 5.2 kDa and terminal thermosensitive 6.8 kDa blocks of PNIPAM as determined by NMR (Figure 1, Table 1).where k is the Boltzmann constant, n the refractive index, and  the viscosity of the solvent.
DLS measurements were performed for solutions of PNIPAM-b-PDMS-b-PNIPAM filtered with a 0.45 PVDF filter into a dust free cuvette.Measurements were repeated three times, and standard deviations were calculated for all measured parameters.The derived scattered intensity Is was calculated from these experiments.

Small-angle neutron scattering (SANS)
SANS experiments were performed at instrument D11 at the Institut Laue-Langevin (ILL) in Grenoble, France.The incident neutrons had a wavelength λ = 6.0 Å with a spread of 9%.
A 3 He gas detector with an area of 96 × 96 cm 2 and a pixel size of 7.5 × 7.5 mm 2 was used.A q-range from 0.0022 to 0.38 Å -1 was covered using three sample-to-detector distances: 1.2, 8, and 20 m. q is the momentum transfer, q = 4π×sin(θ/2)/λ, with θ being the scattering angle.
Samples were mounted in quartz glass cells from Hellma Analytics with a neutron path of 1 mm.At the end of each run, the sample transmission was measured.Boron carbide was used for measurement of the dark current, and H2O was used for the detector sensitivity and calibration of the intensity.The scattered intensity curves were azimuthally averaged and corrected for background scattering from the solvent-filled cell and parasitic scattering.
Scattering from D2O was measured separately and subtracted from the solution scattering data.

SANS SLD calculations
To assess the scattering of newly synthesized PNIPAM-b-PDMS-b-PNIPAM copolymer the SLD values of each block were calculated.The PDMS block has an SLD value of 0.63•10 9 cm -2 , but PNIPAM has a one-order-of-magnitude-higher SLD value of 8.1•10 9 cm -2 .
We expect that in 100% D2O, both blocks will be visible, although scattering from PNIPAM block will dominate over the scattering from PDMS block.

The Beaucage fitting model
The SANS curves in D2O were fitted by the Beaucage model: 33,34,35   () =  (− where G is the Guinier pre-factor of the larger structure, B is a pre-factor specific to the type of power-law scattering, Gs is the Guinier pre-factor of the smaller structure, Bs is a pre-factor specific to the type of power-law scattering, Rg is the size of large-scale structure, Rsub is the surface-fractal cut-off radius of gyration, Rs is the size of small subunits, P is the scaling exponent of the power law assigned to the larger structure Rg, and Ps is the scaling exponent of the power law assigned to the smaller structure Rs.

Synthesis of PNIPAM-b-PDMS-b-PNIPAM
PNIPAM-b-PDMS-b-PNIPAM was successfully synthesised by ATRP from a PDMS macroinitiator, and the structure was confirmed by NMR (Figure S1, supporting information).THF seems to be a suitable solvent for ATRP from PDMS macroinitiators, which is also suitable for many water-soluble monomers.Whilst there are a number of studies which graft PNIPAM to PDMS surfaces to modulate cell-attachment, 37,36 this is the first reported synthesis of this block copolymer, to our knowledge.been used to create nanocomposite paper. 41

Temperature behaviour of PNIPAM-b-PDMS-b-PNIPAM
To evaluate the temperature behaviour of PNIPAM-b-PDMS-b-PNIPAM, dynamic light scattering experiments (DLS) were conducted in aqueous solutions in H2O.The cloud point value (CPT) was determined to be 30.0± 0.5 °C as the onset of a rapid increase in the derived scattered intensity Is (Figure 2a).This CPT value is somewhat lower that the CPT for pure PNIPAM (32 C).This discrepancy is clearly due to the presence of the hydrophobic PDMS block in the copolymer structure.3][44] The incorporation of hydrophilic PEG as a central block creates an opposing trenda CPT value that increases, as was previously observed for PNIPAM-b-PEG-b-PNIPAM copolymer. 27 A peculiar feature can be observed in Figure 2b.The value for Rh of 10 nm is higher than we would expect for a molecularly dissolved polymer at low temperatures below CPT.It is not surprising, however, considering the strong hydrophobicity of the PDMS block.One can expect a preliminary self-organization of copolymers even below the CPT value.Larger structures with low polydispersity indices (<0.1) were observed by DLS at elevated temperatures, as could be expected due to the thermoresponsivity of PNIPAM (Figure S2, supporting information).
SANS experiments could shed light on nanoparticle structures below and above CPT.Several features should be noted for the SANS curves at 25 and 40 °C for the PNIPAM-b-PDMS-b-PNIPAM system in D2O (Figure 3).The curve shows q -3.9 behaviour at 40 °C in a middle q range at 0.017-0.033A -1 .The most spectacular modification is witnessed at middle q range for 25 °C.A scaling exponent value decreases to the value of -2.5.Such finding can imply that the structure of aggregates that exist in solution below CPT is different from the aggregates above CPT.The scaling exponent value close to -4 is known as Porod behaviour, 45 indicating the presence of compact objects with sharp boundaries in solution.

SANS experiments for PNIPAM-b-PDMS-b-PNIPAM
Such findings corroborate with the DLS data described above.At a high q range at 0.065-0.3A -1 , the scattered intensity has more gradual behaviour, with scaling exponents -1.7 and -2.3, for 25 and 40 °C, respectively.Such q dependence is usually attributed to macromolecular q -4.0 I, cm -1 q, A -1 T=25C T=40C q -2.3 chain conformations with excluded volume effects.The upturn at the lowest q visible for the SANS curve at 25 °C could be explained by the presence of fractal aggregates.
To summarize, two different types of structures were revealed by inspecting SANS curves -large objects with sharp boundaries and smaller entities with a coil conformation.To account for this complexity, the Beaucage model was applied. 337][48][49] From the fitting procedure, we can conclude that the sizes of whole mass fractal aggregates at 25 and 40 °C for PNIPAM-b-PDMS-b-PNIPAM are consistent with DLS data -38 and 150 nm (Table 2).The discrepancy could be attributed to different sensitivities of the methods; DLS provides information on Rh, whereas SANS provides Rg.The subunit size Rs also depends on temperature, at 0.9 vs 8.5 nm.

Figure 2 .
Figure 2. Temperature dependence of the scattered intensity Is (a) and volume-weighted

Figure 3
Figure 3 shows the SANS curves obtained for PNIPAM-b-PDMS-b-PNIPAM at two

Figure 3 .
Figure 3. SANS data for the PNIPAM-b-PDMS-b-PNIPAM copolymer at two temperatures, 25 o C and 40 o C in D2O.The red and blue lines are the fits by the Beaucage model.

7 (
the nanoparticle model for the PNIPAM-b-PDMS-b-PNIPAM copolymer can be described as follows: At 25 °C, a nanoparticle of overall radius of 38 nm, consisting of small 5.0 nm particles, which are arranged inside of a fractal with scaling exponent 2.surface fractal).Inside the small particles, they behave as swollen macromolecular coils in good solvent; the scaling exponent is 1.6 At 40 °C, nanoparticles are much larger; Rg of 150 nm.They consist of smaller particles with 12.6 nm particles that are arranged inside of a fractal with a scaling exponent of 4.0 (surface fractal).Inside the small particles, they behave as an almost Gaussian polymer; the scaling exponent is 2.3.CONCLUSIONNovel thermoresponsive copolymers PNIPAM-b-PDMS-b-PNIPAM were synthesised from commercially available precursors using ATRP.Using small-angle neutron scattering, we were able to investigate in detail the internal structure of nanoparticles formed from novel thermoresponsive PNIPAM-b-PDMS-b-PNIPAM triblock copolymer in aqueous solutions.In contrast with previously reported copolymers with more hydrophilic central blocks, both PNIPAM-b-PDMS-b-PNIPAM copolymers form well-defined aggregates at room temperature.The best results were obtained by application of the Beaucage model describing the nanoparticles formed at 40 °C as an aggregate with a two-level hierarchical structure.

Table 2 .
Comparison table of fitting parameters for SANS curves of PNIPAM-b-PDMS-b-