Charged Star Diblock Copolymers in Dilute Solutions: Synthesis

Apr 20, 2015 - Using asymmetric flow field-flow fractionation (AFFFF), Zetasizer, and small-angle X-ray scattering (SAXS), the phase behavior and nano...
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Charged Star Diblock Copolymers in Dilute Solutions: Synthesis, Structure, and Chain Conformations Sara Bekhradnia,†,‡ Jakob Stensgaard Diget,† Thomas Zinn,† Kaizheng Zhu,† Sverre Arne Sande,‡ Bo Nyström,† and Reidar Lund*,† †

Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern, N-0315 Oslo, Norway Department of Pharmacy, University of Oslo, P.O. Box 1068, Blindern, N-0316 Oslo, Norway

‡

S Supporting Information *

ABSTRACT: We present a systematic investigation of a novel series of star polymers consisting of arms made up from poly(Nisopropylacrylamide)-b-poly(2-acrylamido-2-methylpropanesulfonate) (PNIPAAM-block-PAMPS) block copolymers. The polymers were synthesized as a 3-arm and 2-arm (i.e., a tetrablock copolymer) using a “core-first” method and a sequential atomic transfer radical polymerization (ATRP) protocol. Using asymmetric flow field-flow fractionation (AFFFF), Zetasizer, and small-angle X-ray scattering (SAXS), the phase behavior and nanostructure of the system in dilute solutions are studied in detail. While AFFFF equipped with a light scattering and refractive index detectors provides distribution of molecular weight and overall sizes in solution, we use SAXS combined with theoretical modeling to elucidate the inter- and intramolecular interactions of the star polymers. In particular, by employing a detailed model for a star-diblock copolymer assuming Gaussian chain statistics, we extract the chain conformation for each polymer block separately. We find that the radii of gyration, Rg, for both PNIPAAM and PAMPS are very similar to the expected dimension of free chains in solution. By adding salt, we show that the strong interstar repulsion found in water is dramatically reduced after adding as little as 0.025 M NaCl. Further increase of NaCl up to 0.2 M shows that the system essentially behaves as neutral polymers in a good solvent. Concerning the chain conformations, addition of NaCl seems to have a small effect on the Rg of the different blocks.

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“core-first” approach in employing ATRP or RAFT. In the former “arm-first” method, linear polymer arms are first synthesized and then cross-linked together by adding a crosslinking agent15,16 or difunctional monomers.17,18 Alternatively, star polymers can be synthesized by letting the linear polymer chains grow from a core with multiple initiation sites. This method can be preferable over the “arm-first” approach as it provides better control of the number of arms provided that all initiation sites are equally activated. Conformation and overall structure of polymer stars can be conveniently studied using solution scattering methods, in particular light scattering or small-angle X-ray or neutron scattering (SAXS/SANS). While dynamic light scattering

INTRODUCTION Advances in modern synthetic polymer chemistry provide the possibility to precisely control the architecture, molecular weight, and composition of polymer structures. Controlled polymerization methods such as atom transfer radical polymerization (ATRP)1−3 or reversible addition−fragmentation chain transfer (RAFT)4,5 techniques allow the synthesis of polymeric materials with highly controlled and designed properties to be useful for many applications, ranging from advanced coating materials6,7 to nanocarriers for biomedical applications.8,9 Control of polymer chain architecture is interesting for accurate tuning of properties both in bulk (viscosity and flow behavior),10,11 surfaces (adsorption and cell interactions),12 and in solutions (colloidal interactions)13,14 where stability and interactions are key features when used as, e.g., drug carriers. Star polymers represent a simple type of branched polymer systems and may be synthesized using either an “arm-first” or © XXXX American Chemical Society

Received: December 10, 2014 Revised: March 29, 2015

A

DOI: 10.1021/ma502488u Macromolecules XXXX, XXX, XXX−XXX

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PAAM-b-PAMPS) by sequential addition of NIPAAM and AMPS monomers on well-defined multifunctional initiator cores (2 and 3). Furthermore, we investigate in detail the nanostructure of the star polymers with a particular focus on the chain conformation and the effect of electrostatic interactions using SAXS. The latter is elucidated by adding salt and performing systematic and zeta-potential experiments and SAXS experiments combined with theoretical modeling. Using asymmetric flow field-flow fractionation (AFFFF), we characterize the overall size and aggregation behavior in dilute solutions.

(DLS) gives access to the global dimensions, SAXS/SANS methods measure density variation on the length scales of a few nanometers up to about 100 nm. SAXS/SANS techniques are particularly useful to study polymer systems, including star polymers, as the wavelength is suitable to study both the intraand intermolecular correlations of the system, giving information on internal structure, conformation, overall shape, and interparticle interactions. The conformation of neutral star polymers has been studied extensively in the 1980−1990s where both theoretical and experimental studies suggest that the local structure of stars is reminiscent of a semidilute polymer solution with radially increasing blob size.19 Using SANS, it was shown that the conformation of the individual arms in neutral star polymers or “starlike” block copolymer micelles are more stretched as compared to linear chains20,21 due to enhanced intramolecular repulsive interactions, which induce stretching on the expense of loss of conformational entropy. Furthermore, upon increasing the number of arms, star polymers resemble soft spherelike structures, in terms of both structure20 and interactions22 between stars. Hence, star polymers are interesting as a system in soft condensed matter physics as they link the fields of classical polymer and colloid physics.23 Whereas neutral stars are rather well understood as a result of the considerable efforts over the past decades, less is known about their charged counterparts, i.e., polyelectrolyte stars24−27or star block copolymers,28−30 i.e., stars with arms consisting of diblock copolymers. Star-branched polyelectrolytes are interesting as nanocarriers and complex forming agents,31,32 where their charges along the backbone also facilitate enhanced colloidal stability. Moreover, the interactions and chain conformation can be altered through their environmental conditions, such as salt concentration, type of counterions, pH, etc.33 The structural properties of polyelectrolytes arise from the interplay between factors such as electrostatic repulsions, chemical connectivity, translational entropy of the counterions, backbone hydrophobicity, etc.24,25,34,35 Recent SAXS studies have demonstrated the strong correlations imposed by charges that lead to pronounced structure factor peaks in the scattering data.36,37 Further control of the structure and conformation of star polymers can be achieved by inclusion of thermally responsive polymers such as poly(N-isopropylacrylamide) (PNIPAAM)38−40 or poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA).41−43 In an interesting study on PDMAEMA star polymers, it was shown that the polybasic polymer that is a polybase is unimolecularly dissolved at ambient temperatures but self-assembles into a controlled moderate size upon heating43 due to residual charges at neutral pH (pKa > 7). Hence, electrostatic charges may serve as means of controlling the nanostructure in thermoresponsive star-polymer systems. Stimuli-responsive star-polymers have attracted a great deal of interest in recent years due to their unique properties compared to their linear counterparts.44 These star polymers have the ability to respond to the various stimuli like pH45 and temperature.46 These features and the special architectures make them suitable for a broad range of applications, such as drug delivery and biotechnology.47 Herein we report on the synthesis and characterization of charged, thermoresponsive, star diblock polyelectrolytes 3-arm as well as a 2-arm, i.e., linear tetrablock copolymer. We employ a “core-first” ATRP-based method to synthesize star polymers with a diblock arm consisting of poly(N-isopropylacrylamide)block-poly(2-acrylamido-2-methylpropanesulfonic acid) (PNI-

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EXPERIMENTAL SECTION

Synthesis and Characterization. All materials were purchased from Sigma-Aldrich with the highest quality, and they were used without further purification, unless otherwise stated. N-Isopropylacrylamide (NIPAAM, Acros) was recrystallized twice in a toluene/hexane mixture and further dried under vacuum before use. The charged monomer 2acrylamido-2-methyl-1-propanesulfonic acid sodium salt, abbreviated as AMPS (50 wt % in H2O, Aldrich), was purified from the trace inhibitor present in the sample by removing most of the water in the vacuum oven at 60 °C, followed by washing with cold ethanol and finally drying under vacuum. N,N,N′,N″,N‴,N⁗-(Hexamethyltriethylenetetramine) (Me6TREN) was synthesized according to the literature.48 Di- and trifunctional core initiators were prepared by reacting the multiple hydroxyl compounds (2,2′-biphenyldimethanol and triethanolamine) with 2-bromoisobutyryl bromide in the presence of triethylamine.2 The 1 H and 13C NMR data of these synthesized “core” compounds are collected in the Supporting Information. Polymerization. The 2- and 3-arm PNIPAAM-b-PAMPS derivatives were prepared via a simple “one-pot” two-step ATRP procedure under similar conditions as described in detail previously (cf. Scheme 1)49−55 and explained in more detail in the Supporting Information.

Scheme 1. Synthetic Route for the Preparation of the StarPNIPAAM-b-PAMPS Block Copolymers via the “One-Pot” Two-Step ATRP Procedure

Briefly, the polymerization reaction was performed in a water/DMF (40:60, v/v) solvent mixture at 25 °C, and the initiator/catalyst system in the mixture contained CuCl, CuCl2, Me6TREN, and the core initiator (with molar feed ratio ([NIPAAM] = 1 M, [NIPAAM]/[AMPS]/ [initiation sites]/[CuCl]/[CuCl2]/[Me6TREN] = 50/30/1/1/0.6/ 1.6). Since the conversion of NIPAAM was not complete, some NIPAAMs were incorporated in the second block. Graphical representations and chemical structure of the 2- and 3-arm block copolymers are depicted in Figure 1. The preparation and purification procedures of the polymer were carried out under similar conditions as described in detail previously.4−6 Characterization. The chemical structure and composition of the star-PNIPAAM-b-PAMPS derivatives were all ascertained by their 1H NMR spectra in heavy water (Bruker Avance AVI600 MHz, TSP (3(trimethylsilyl)propionic-2,2,3,3-d4 sodium salt) as the reference) (Figure 2). The total number of repeating units of NIPAAM/AMPS B

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Figure 1. Chemical structure of the multifunctional cores/initiators (2- and 3-) and their PNIPAAM-block-PAMPS “star” block copolymer derivatives. Top: graphical representation. Bottom: chemical structure of the block copolymer arms. methylene, N(CH2CH2O)3). The results of these PNIPAAM-b-PAMPS star copolymers are collected in Table 1. The second monomer (AMPS) was added to conduct the further chain extension when the conversion of the first monomer (NIPAAM) reached 85−90%. Usually the reaction time was kept between 28 and 30 min. To monitor conversion, a small amount of the reaction mixture was withdrawn, diluted with D2O, and analyzed using 1H NMR (comparing the integral of the decreasing vinyl signals at 5.5−6.0 ppm with the growing polymer main chain at 1.4−2.2 ppm). The chemical structures based on NMR were found to be [(PNIPAAM107-b-P(NIPAAM0.14-co-AMPS)35]2 for the 2-arm copolymer and [(PNIPAAM107-b-P(NIPAAM0.17-co-AMPS)44]3 for the 3-arm copolymer. The composition is based on the monomer conversion of the NIPAAM and AMPS species (calculations based on NMR carried out before addition of AMPS, i.e., first block, and after termination of the polymerization, i.e., complete polymer). We found that the second block of each arm contained about five NIPAAM residues for the 2-arm star block copolymer and seven NIPAAM residues remained for the 3-arm star block copolymer. However, for simplicity, we will refer to the polymers as 2- and 3-arm star (PNIPAAM-block-PAMPS) block copolymers in the remainder of the paper. Asymmetric Flow Field-Flow Fractionation. The numberaverage molar mass (Mn), weight-average molar mass (Mw), and polydispersity index (Mw/Mn, PDI) of the 2- and 3-star block polymers, were measured using AFFFF. The AFFFF experiments were conducted on an AF2000 FOCUS system (Postnova Analytics, Landsberg, Germany) equipped with an RI detector (PN3140, Postnova) and a multiangle (seven detectors in the range 35°−145°) light scattering detector (PN3070, λ = 635 nm, Postnova). The field-flow fractionation channel was installed with a 350 μm spacer and a regenerated cellulose

Figure 2. 1H NMR spectra of the synthesized star polymers in D2O (600 MHz, 25 °C). per arm in the star-PNIPAAM-b-PAMPS was calculated from the integral values of the characteristic peak of NIPAAM (δ = 3.91 ppm, −CH(CH3)2, 3), the characteristic peak of AMPS (δ = 3.38 ppm, −CH2SO3Na, 8), and the typical peak of the different functional core/ initiator (difunctional core; δ = 7.2−7.6 ppm, peak 9 of the biphenyl proton and the trifunctional core; δ = 4.2 ppm, peak 10 of the C

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Macromolecules Table 1. Data of the Star-(Poly(NIPAAM)-block-poly(AMPS)) Obtained by 1H NMR and Asymmetric Flow Field-Flow Fractionation

a

no. of arms

units of monomers per arma

Mn(1H NMR) (103, g/mol)

Mn (AFFFF) (103, g/mol)

Mw (AFFFF) (103, g/mol)

PDI (Mw/Mn, AFFFF)

2 3

[(PNIPAAM107-b-P(NIPAAM0.14-co-AMPS)35] [(PNIPAAM107-b-P(NIPAAM0.17-co-AMPS)44]

33.4 41

29.7 44.2

41.8 70

1.41 1.58

Units of NIPAAM and AMPS were calculated from the corresponding 1H NMR spectra.

membrane with a cutoff of 10 000 g/mol (Z-MEM-AQU-425N, Postnova). A 20 μL sample was injected through an injection loop. The refractive index increment, dn/dc, was obtained using the RI detector in a separate run employing only the tip flow, hence excluding separation, possible interactions with the membrane, salt gradients, and polymer escaping through the membrane. Assuming 100% mass recovery, the values of dn/dc for the 2- and 3-arm polymers were found to be 0.1393 and 0.1353 mL/g, respectively. Molar mass measurements were conducted with a constant detector flow rate of 1 mL/min. The focusing time was 5 min at a cross-flow of 3 mL/min. After the focus step, the cross-flow was linearly decreased to 0 mL/min over 12 min, followed by 8 min of elution using only the tip flow at 1 mL/min. To minimize aggregation, the measurements were carried out with a channel temperature of 10 °C. Samples were prepared immediately prior to analysis by mixing the polymer and cold 0.1 M NaCl in a flask that was placed in an ice/water mixture. The 0.5 wt % samples were filtered through 0.45 μm syringe filters and kept stirred and cold prior to use. The data were analyzed by using the Postnova software (AF2000 Control, version 1.1.025). Values of Mw of the samples in very dilute solutions were determined using the Zimm-type fit. A graphical representation of the molar mass distribution (differential distribution) and the characteristic data of the star block polymers can be found in Figure 3 and Table 1, respectively. A shoulder is seen at

mass were analyzed. Less than 5% of the total injected mass was for both polymers aggregated species. Zeta-Potential Experiments. The instrument that was employed in this study is a Zeta-sizer Nano ZS instrument, MAL1049741, made by Malvern instruments Ltd., United Kingdom. The sample cell that was used is a “dip” cell, including palladium electrodes with 2 mm spacing, one PCS1115 cuvette, and cap. The experiments were carried out at 25 °C with a polymer concentration of 1.25 mg/mL. The values of the electrophoretic mobility UE were converted to ζ potential values by employing the Henry equation, UE = 2εζf(Ka)/3η, where the viscosity (η) and dielectric constant (ε) of water without salt was used. The Smoluchowski approximation to Henry’s function ( f(Ka) = 1.5) was utilized. Three runs were carried out, and the average value from these runs was reported. Small-Angle X-ray Scattering (SAXS) Experiments. The synchrotron SAXS experiments were performed on the bio-SAXS high-throughput P12 EMBL beamline located on the PETRA III storage ring at DESY, Hamburg. The instrument is equipped with Pilatus 2M detector, and the measurements were carried out in a Q-range of 0.0076−0.46 Å−1. The data acquisition was executed under injection of a 10 μL amount of sample into quartz capillaries (2 mm) using 20 successive frames with 50 s exposures that were later added to improve the statistics. No sign of beam radiation damage was observed under these conditions. The data were averaged after normalization to the intensity of the transmitted beam and calibrated on an absolute scale using Millipore water as a primary calibrating standard. Theoretical Modeling of SAXS Data. The first description of the small-angle scattering from star-polymers is due to Benoit,̂ who deduced the from factor of a star consisting of f chains following Gaussian chain statistics.56 Later, a generalized ad hoc model was derived by Dozier et al. to describe the structure of star polymers dissolved in good solvents.57 The static form factor for star polymers was calculated in the full excluded-volume limit with the aid of renormalization group techniques by Alessandrini et al. in 1992.58 Although, the latter renormalization group approach provide a more realistic approach, a general framework to easily implement excluded volume effects for diblock copolymer star structures does not exist. Therefore, the model for an AB-diblock star polymer is readily considered as a collection of f Gaussian chains that are connected to a common star center. Each branch consists of the inner Bblock with a radius of gyration RgB, while the A-blocks with a radius of gyration RgA are connected to the ends of the B-blocks (see Figure 4). In the analysis we disregard the small core, which constitutes only a minor part of the total molecular weight. P(Q) can be calculated by explicitly counting the self- and cross correlation terms between the A/B blocks by using the chain center and connection points. Assuming Gaussian chain statistics and the general approach proposed by Svaneborg and Pedersen,59 the form factor can be calculated by explicitly considering a B-A diblock connected at the chainend of the B block. Alternatively, the form factor can be calculated explicitly by counting the correlations as done by Huber et al.60 The result for the form factor is

Figure 3. Differential (closed symbols) and cumulative (open symbols) molar mass distribution of the 2- and 3-star diblock polymers obtained by AFFFF. See Table 1 for Mw, Mn, and PDI values.

lower molecular masses, most likely revealing chain-terminated species. The elution diagrams (not shown here) revealed the presence of some aggregates for both polymers at 10 mg/mL. However, this was less pronounced at 2.5 mg/mL, which was later used, in the final analysis. This may be attributed to the fact that the MALLS and RI detectors operate at 27.5 and 32 °C, respectively, which may induce association between star PNIPAAM-rich star-polymers. Data shown for the 2- and 3-star block polymers in Figure 3 and Table 1 are from analysis of the complete star block copolymer peak, excluding the aggregated species. For the 2- and 3-star block polymers 84 and 86% of the total injected

P(Q ) =

D

βB2[FB(Q ) + (f − 1)AB2 (Q )] f (βA + βB)2

+

βA 2[FA(Q ) + (f − 1) ψB2(Q ) AA (Q )2 ]

+

2βA βB [1 + (f + 1) ψB(Q )] AA (Q ) AB(Q )

f (βA + βB)2 f (βA + βB)2

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Figure 4. Illustration of the star diblock structure. The scattering functions are calculated by assuming that each block can be treated as a Gaussian subchain, and there is no correlation between the different blocks/arms beyond the connectivity. The first and the second terms describe the self-correlation between all the A and B blocks, respectively, while the third term describes the contribution because of the cross-correlation between A and B segments. Note that P(Q=0) is normalized to unity. Here f is the number of arms, βi = Vi(ρi − ρ0), where Vi and ρi are the volume and the scattering length density, respectively, of the polymer block (i = A, B). Vi was calculated from the solution density di according to Vi = Mi/di, where Mi is the molecular weight of the polymer block. ρi = Zibe/Vi where Zi and Vi are the number of electrons and specific volume, respectively, and be is the scattering length of an electron (Thomson radius). The scattering length density of the solvent is given by

where x = (QRg,B)2and the radius of gyration Rg,star 2 = (3f − 2)/f)Rg,B2. The total scattering for a diblock star polymer is then given by

I(Q )star‐diblock =

I(Q )tot =

where MH2O and cNaCl are the molecular weight and molar concentration of NaCl, respectively, and NA is Avogadro’s constant, while ρs,solvent is the measured density of the NaCl/water solvents. The form factor for a single Gaussian chain polymer with radius of gyration, Rgi, was derived by Debye:61

I(Q )tot =

(3) 62

i 2

Here xi = (QRg ) . For the scattering amplitude of Gaussian chain we have

A(xi) =

1 − exp(− xi) xi

The phase factor is given by

(4)

(5)

I(Q )star‐diblock 1 + 2A 2 cMstarP(Q )

(9)

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A simple test of the validity of eq 1 is to make the A-block invisible, i.e., contrast matching with solvent (βA = 0). Under this condition the classical form factor for a regular Gaussian star calculated by Benoit56 is recovered: Fstar(x) =

(8)

where A2 is an effective second virial coefficient that is positive for repulsive interactions, and for theta conditions it tends to zero. Mstar is the total molecular weight (Mw) of the star polymer. The composition and molecular weights were taken to from the NMR and A4F results. In the final fits the free parameters were Rg of the PNIPAAM and PAMPS block of the arm. Based on a preliminary fit analysis, the solution density of PAMPS was found to be about 1.25 g/cm3. The latter could not be determined with precision based on the solution density of the star polymers and was considered as a fit parameter.

40,62

Ψ(xi) = exp(− xi)

ϕ f (β + βB)2 P(Q ) S(Q ) Vtot A

Here φ is the volume fraction, f is the number of arms, and Vtot is the total volume of the star polymer. For the structure factor describing the strong repulsive interactions between the stars, we used the rescaled structure factor S(Q) calculated for screened Coulombic interactions.63 The fitted parameters were the effective hard-sphere radius, RHS, the total charge, Z, and the effective volume fraction, η. For the latter we used a common scaling factor for all concentrations such that η = Cscalingφ, where Cscaling is a numerical fit parameter and φ is the experimental volume fraction. In the case of weaker interactions, a virial type expansion can be used:

(2)

2 [exp(− xi) − 1 + xi] xi2

(7)

In order to take into account a possible correlations between the stars, i.e., electrostatic repulsions, a structure factor, S(Q), must be included:

⎡ 1 ⎤ (ρd,solvent − MNaClc NaCl)Z H2O + c NaClZ NaCl ⎥NAbe ρ0 = ⎢ ⎢⎣ M H2O ⎥⎦

F(xi) =

ϕ f (β + βB)2 P(Q ) Vtot A

RESULTS AND DISCUSSION Small-Angle X-ray Scattering (SAXS): Interactions, Nanostructure, and Chain Conformations. Structure and Interactions in Water without Salt. Figure 5 depicts the normalized absolute scattering intensity, dΣ/dΩ(Q), as a function of the modulus of the momentum transfer vector, Q,

⎤ f−1 2 ⎡ x − (1 − exp(− x)) + (1 − exp(− x))2 ⎥ 2⎢ ⎦ ⎣ 2 fx (6) E

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Figure 6. Zeta-potential as a function of salt concentration for the 2- and 3-arm stars at 25 °C and a polymer concentration of 1.25 mg/mL.

conformation of the stars, we used the scattering model for a generalized block copolymer star obeying Gaussian statistics described in the Theoretical Modeling section. In the analysis, we have included a Hayter−Penfold structure factor that describes the strong repulsive interactions between the star polymers. As illustrated in Figure 5, we obtain rather good fits using this simple Gaussian diblock star model. The fitted line describes the data rather well at low Q, including the structure factor contribution. During the fit analysis, we performed simultaneous fits of the scattering data for all concentrations, where a common set of parameters were used and assumed independent of polymer concentration. These include the radii of gyration of each block and the density of PAMPS, Cscaling controlling the effective volume fraction of star polymers (η = Cscalingφ), the effective charge per entity, Z, and the effective radius, RHS. In addition, the salt concentration, i.e., the effective ionic strength determining the screening length, was held constant to an arbitrary small value of 1 × 10−4 M. In all cases the concentration dependence was included only in the effective volume fraction. From the least-squares fit analysis we obtain a radius RHS of 68 and 61 Å for the 2- and 3-arm star, respectively. The effective total charge of Z = 12 and 14 for 2- and 3-arms, respectively. The fit analysis of the absolute intensity suggests that most sodium ions are rather closely associated with the star-polymer backbone. However, this cannot be determined on the basis of these data alone and would require further analysis e.g. by anomalous SAXS (ASAXS).65 For the effective volume fraction we obtain a value about Cscaling = 14 for both polymers. It is interesting to note that there is no clear systematic deviation of the scattering model expected from the assumption of Gaussian chain statistics. Water is indeed expected to be a good solvent for both PNIPAAM and PAMPS chains at room temperature; thus, excluded volume effects should be present. However, given the good agreement with the data and the lack of detailed scattering models that take into account correlation between charged segments, we chose to proceed with the present model. From the model fits we obtain Rg of 39 ± 5 and 40 ± 4 Å for the PNIPAAM inner block for the 2- and 3-arms, respectively. For the outer PAMPS block, the corresponding values are Rg = 16 ± 3 and 17 ± 2 Å for the 2- and 3-arm star copolymer, respectively. The values for the two different blocks seem to be consistent with the dimensions calculated from chain statistics of a polymer in a good solvent and the NMR data. First considering PNIPAAM, we may estimate the radius of gyration from literature values for linear chains according to66 Rg = 0.158Mw0.58

Figure 5. Small-angle X-ray data of (a) 2-arm and (b) 3-arm star polymers at various concentrations in the dilute regime. The data were recorded at 25 °C and normalized to concentration. The solid lines display fits to the Gaussian diblock star model including a structure factor describing screened Coulomb interactions (see text for details).

of 2-star and 3-star PNIPAAM-block-PAMPS copolymers at different concentrations. First turning our attention to the features at high Q, we observe a characteristic diffuse scattering pattern that very much resembles that of polymer chains. Analyzing the slope, we obtain a characteristic Q dependence that can be roughly described with a power law, Q−x, where the exponent, x, taking values of about 1.6−1.8. This is slightly less than what is expected from a random Gaussian chain for which x = 2 is expected but in line with the expectation from chains exhibiting excluded volume statistics, x = 1/0.588 = 1.7. Hence, the internal structure of the star polymers is reminiscent of swollen polymer chains. Now, considering the scattered intensity at low Q, both the 2and 3-arm polymers show a pronounced peak at low Q values. The structure factor peak moves toward higher Q with increasing concentration, demonstrating the repulsive interparticle interactions due to the highly charged nature of the star polymers (cf. also Figure 6). The inset plots of Figure 5 depict the scaling of the peak maxima, obtained through manual inspection, as a function of the concentration. The results show a scaling of Q* ∼ cx, where x ≈ 0.36 and 0.44 for the 2- and 3-arm, respectively. This scaling exponent is slightly larger than what is expected in dilute concentrations of polyelectrolytes64 (x ≈ 1/3), i.e., at concentrations below the overlap concentration c*, which can be roughly estimated from c* ≅ 3f Varm/4πRg3, where Varm is the volume of one arm, is found to be about 10 vol % for the present system. This might be related to partial screening or the “softness” of the star polymers. At higher Q, we observe that the data collapse and the scattering profile exhibit a more diffuse polymer-like scattering. To describe the data and extract information concerning detailed F

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Figure 7. Effect of salt addition on the scattering patterns of 2- and 3-arm star polymer at different polymer concentrations. The solid lines display fits of the Gaussian diblock star model, where a virial expansion is included to take into account the repulsive interaction between the stars (see text for details).

Å, which gives 37 Å for the current molecular weight. Comparing

Rg2 = −

with the experimental values an excellent agreement is found, Rg = 36−37 Å in water. For the PAMPS-rich block we may provide a rough estimate using Rg ≈ leffNmon

0.588

3 d2P(Q ) 2 dQ 2

Q =0

2

= [3f (βA + 3βA βB + 2βB 2)RA 2 − 2(βA 2 + 3βA βB

1/2

/6 , where Nmon is the

+ 3βB 2)RA 2 + 3f (βA + βB)βBRB 2 − 2βB 2RB 2]

number of monomers and leff is the effective segment length.

/[3f (βA + βB)2 ]

Assuming that latter segment length of PAMPS is 5 Å, we obtain Rg ≈ 18 and 20 Å for the 2- and 3-arm, respectively, which is

(10)

In this way we obtain Rg = 51 and 56 Å for the 2- and 3-arms, respectively. Comparing with the results from the AFFFF analysis using a multiangle light scattering (MALS) detector, we obtain larger values of Rg ≈ 89 and 99 Å for 2- and 3-arms, respectively. The higher values from MALS again indicate the presence of larger entities, which are less visible in the SAXS experiments. However, it should be pointed out that the accuracy

compatible with our experimental data demonstrating that the fit analysis and the form factor descriptions are reasonable. From the individual radius of gyration for each arm, we may also calculate the total overall Rg of the whole star. This is a rather straightforward determination from the form factor (eq 1) G

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Macromolecules of Rg values from MALS is rather limited given the small values (Rg < λ/ 20 ≈ 30 nm, where λ is the laser wavelength). In the subsequent section, we investigate the role of electrostatic interactions on the conformation of the 2- and 3arm star polymers by adding salt. Effect of salt on the zetapotential, ζ, for the two star polymers will also be illustrated in the section below. Effect of Salt Addition. Figure 6 depicts the measured zetapotential as a function of salt concentration for the two starpolymers. We notice that in the absence of salt, the value of ζ is higher for the 3-arm copolymer than the 2-arm, which might be ascribed to a higher effective charge density. It is interesting to observe that even at a quite low salinity, the electrostatic interactions are screened. This suggests that the polymers will exhibit properties that approach that of nonionic polymers. At about 0.20 M salt concentration, there is no difference in behavior between the two polymers. In Figure 7, the scattering curves for the 2- and 3-arm polymers are shown for different polymer concentrations in the presence of different amounts of salt. It is very clear from the curves that even a small amount of salt of about 0.025 M is sufficient to screen the electrostatic interaction sufficiently to avoid a clear structure factor peak (correlation hole) that was found in pure water. From the composition obtained from NMR, one can calculate that the total amount of charged groups is of the order of 0.01 M at 5 mg/ mL total polymer concentration. Hence, addition of only 0.025 M NaCl is sufficient to match the charges even at 10 mg/mL. However, it is also clear from the data that the intensity at low Q decreases systematically as a function of salt content. This demonstrates the existence of residual repulsions that resembles that of polymers in good solvents, where the excluded volume effect can be described in terms of an effective second virial coefficient representing pair interaction. Hence, the interstar interactions can then approximately be taken into account by using the virial expansion given in eq 9. The results of the fit analysis are shown as red lines in Figure 7. The pronounced screening of the electrostatic interactions at low salinity is supported by the results from the ζ-potential measurements. At larger Q we see rather clear power laws that in all cases are close to Q−1.7 independently of the salt concentrations. As seen, the fitted lines seem to describe the data quite well, again suggesting that the Gaussian star model is a reasonable description of the system. From the fits we obtain the effective virial coefficient, A2, and the radius of gyration for each block of the arms, which both are shown in Figure 8. As seen in Figure 8, the virial coefficient for the 3-arm star is consistently higher than for the 2-arm star polymers. Since the A2 is proportional to the excluded volume, vexcl, this can most likely be attributed to the larger effective volume spanned by an additional arm. Furthermore, from scaling theories,24 vexcl is expected to be inversely proportional to the salt concentration, cs, i.e., vexcl ∼ cs−1. A comparison using a simple parametrization, A2 = A02 + C/cs, is shown in Figure 8a as solid lines. As seen, there is a fair correspondence for both star polymers. The effect of salt can thus be regarded as a reduction in effective volume or, equivalently, an increase in the Debye−Hückel screening length. Since the ζ-potential at a given salt concentration is higher for the 3-arm than the 2-arm star polymer (see Figure 6), better thermodynamic conditions are expected for the 3-arm polymer. Now focusing on the radii of gyration for each block of the arms, we observe that Rg (PAMPS) remains almost constant at all salt concentrations (Figure 8b). At a first glance, this may seem rather counterintuitive since addition of salt should reduce the

Figure 8. Fit parameters obtained by analyzing the data using detailed diblock star polymer model. (a) The virial coefficient and (b) the radii of gyration for each block constituting the star polymer.

intramolecular segmental repulsions. However, this may be attributed to the rather low molecular weight of PAMPS (35−44 units) and the fact that the polymer is likely to exhibit rather strong excluded volume effects due to the connectivity of stars that also impose strong intramolecular correlations. From scaling theories,24,25 balancing the osmotic pressure of counterions, and the elastic chain deformation, predicts that the overall size scales very weakly with the salt concentration, R∼ cs1/5. For PNIPAAM we also observe a near constant Rg at all salt concentrations. A slight contraction in Rg would be expected from a reduction of the cloud point upon addition of NaCl.67 Upon addition of salt, the value of Rg (PAMPS) is virtually unaffected.

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CONCLUSIONS In summary, we have synthesized and thoroughly characterized the solution behavior of charged 2- and 3-arm star copolymers using asymmetric flow field-flow fractionation (AFFFF) and small-angle X-ray scattering (SAXS). In this study, our attention has been focused on the structural features of 2- and 3-arm star polymers. Using SAXS, combined with a detailed theoretical model, we show that we can obtain very detailed information on the interactions as well as the chain conformation of the individual blocks constituting the star polymer. The results show that the dimensions of the chains represented by Rg are rather similar to what can be expected for the free polymer chains. Rather surprisingly, although the effect of small additions of salt significantly reduced the interstar repulsions, the effect in the chain conformation was rather weak. Upon addition of salt, only a slight decrease of Rg with respect to that in water was observed for the PNIPAAM block in the 2-arm star copolymer, while the charged PAMPS block remained almost constant. However, since the scattering contrast is rather similar and small for both blocks, this could be more directly addressed using small-angle neutron scattering (SANS) and partial deuteration as, e.g., was used to deduce the structure factors in starlike block copolymer systems.68 This problem should also be addressed theoretically H

DOI: 10.1021/ma502488u Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

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by means of computer simulations, which may provide further insight into the conformational properties of these rather complex systems.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details; Figures S1−S5. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (R.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The EMBL, Hamburg, is gratefully acknowledged for allocation of the beamtime at the bioSAXS P12 instrument, Petra III. We are grateful to Dr. Petr Konarev for kind help and assistance during the SAXS experiments. T.Z., R.L., and B.N. greatly acknowledge a grant from the Norwegian Research Council, SYNKNØYT (project numbers 218411 and 228573). The research leading to these results has received funding from the EEA/Norwegian Financial Mechanism 2009-2014 under Project Contract No. 7F14009.

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DOI: 10.1021/ma502488u Macromolecules XXXX, XXX, XXX−XXX