Poly(butyl methacrylate-g-methoxypoly(ethylene glycol)) and poly(methyl methacrylate-g-methoxypoly(ethylene glycol)) graft copolymers: preparation and aqueous solution properties

Journal of Colloid and Interface Science 262 (2003) 548–559 www.elsevier.com/locate/jcis

Poly(butyl methacrylate-g-methoxypoly(ethylene glycol)) and poly(methyl methacrylate-g-methoxypoly(ethylene glycol)) graft copolymers: preparation and aqueous solution properties Adrian Horgan,a Brian Saunders,a,1 Brian Vincent,a,∗ and Richard K. Heenan b a School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS, UK b ISIS Facility, Rutherford Appleton Laboratory, Didcot, Oxon, OX11 0QX, UK

Received 14 May 2002; accepted 27 February 2003

Abstract A series of water-soluble, amphiphilic graft copolymers has been prepared by free-radical copolymerization of methoxypoly(ethylene glycol) macromonomers, with either methyl methacrylate or butyl methacrylate as the comonomers, in water/ethanol solvent mixtures. Lower molecular weight copolymers were obtained by increasing the concentration of the initiator, azobisisobutyronitrile (AIBN), used in the polymerization reaction. However, the route used also led to the formation of significant quantities of tetramethylsuccinodinitrile, a toxic byproduct resulting from the cage reaction of AIBN. Static fluorescence measurements using pyrene as a probe, along with 1 H NMR experiments, showed that the graft copolymers form aggregates in water at very low concentrations (∼0.01 g l−1 ) with the pendant hydrophilic graft chains forming a stabilizing shell around the hydrophobic backbone. An increase in the hydrophile–lipophile balance of the graft copolymers was found to lead to smaller aggregates with lower aggregation numbers and highly swollen hydrophilic shells, as revealed by small angle neutron scattering (SANS).  2003 Elsevier Science (USA). All rights reserved. Keywords: Graft copolymer; Micelle; Aggregation; MPEG; Butyl methacrylate; SANS; DLS; Fluorescence

1. Introduction Amphiphilic graft copolymers are of interest because they have many industrial applications and may be synthesized more easily and affordably than block copolymers. Due to the perceived lack of control over the free-radical polymerization process and the range of defect structures and other structural irregularities that are possible, most studies to date of amphiphilic copolymers have featured block copolymers prepared by living polymerization mechanisms. Such copolymers may be obtained with high purity, well-defined architectures, and narrow-molecular-weight distributions, thus facilitating data interpretation. The few studies featuring amphiphilic graft copolymers resembling those described here, that is, having acrylate or methacrylate backbones and poly(ethylene gylcol) (PEG) * Corresponding author.

E-mail address: [email protected] (B. Vincent). 1 Current address: Manchester Materials Science Centre, University of

Manchester and UMIST, Grosvenor St., Manchester, M1 7HS, UK.

branches, have largely ignored the micellization behavior of these types of copolymer in water and the properties of the micelles (e.g., size, shape). Instead, they have addressed aspects of their synthesis and purification and their potential as emulsifiers [1–3]. The purpose of the work described here is to obtain a greater understanding of the micellization behavior of these copolymers and determine the criteria for obtaining small micelles. As is shown in the accompanying paper [4], small graft copolymer micelles may be used as templates to prepare ultrafine, sterically stabilized polymer nanoparticles. The use of such small particles in surface coatings, compared to latex particles of more conventional size, should lead to films with improved optical and mechanical properties. In the current work, in order to obtain small micelles, low-molecular-weight copolymers, having a high hydrophilic content, have been prepared. For similar polystyrenealt-maleic anhydride-g-poly(ethylene glycol) copolymer micelles it has been demonstrated, using transmission electron microscopy (TEM) and static light scattering (SLS) [5], that the aggregation number of the copolymer micelles varies

0021-9797/03/$ – see front matter  2003 Elsevier Science (USA). All rights reserved. doi:10.1016/S0021-9797(03)00239-X

A. Horgan et al. / Journal of Colloid and Interface Science 262 (2003) 548–559

inversely with the grafting density of PEG side chains, in accordance with theoretical predictions made by Förster et al. [6]. TEM is not amenable to the study of amphiphilic graft copolymer micelles of the type described here because of the low glass transition temperature (Tg ) of the micelle core, the poor contrast of the micelles against the organic background, and their instability under the electron beam. Therefore, in the present study other techniques have been used to characterize the micelles, including small angle neutron scattering (SANS) and dynamic light scattering (DLS). Particular attention in this study has been paid to one of the copolymers prepared, as this proved to be a suitable template for the subsequent polymerizations described in the accompanying paper [4].

2. Materials and methods 2.1. Materials Butyl methacrylate (BMA) (Aldrich, 99%) and methyl methacrylate (MMA) (Aldrich, 99%) were distilled over calcium hydride (−40 mesh) under vacuum at room temperature. Pyrene (Aldrich, 99%) was purified by repeated recrystallization from ethanol and subsequent sublimation. Tetramethylsuccinodinitrile was purified by recrystallization from ethanol. 2,2 -azo-bis(2-methylpropionitrile) (AIBN) (BDH, 97%), cetylpyridinium bromide (BDH, 98%), isobutylmercaptan (Aldrich, 92%), ethanol (BDH, AnalaR grade), deionized water (Milli-Q), and methoxypoly(ethylene glycol) 350 methacrylate (MPEG350MA) (Bisomer, kindly supplied by ICI Paints PLC, Slough, Berks, UK) were used without further purification. A Spectrapor cellulose membrane (BDH) with a molecular weight cut-off of 12,000– 14,000 Dalton was used for dialysis. 2.2. Preparation of graft copolymers Poly(BMA-g-MPEG) graft copolymers and poly(MMAg-MPEG) graft copolymers may be synthesized by several routes, including (i) polymerization of monomers from initiating sites on a suitable polymeric backbone, (ii) grafting of preformed reactive polymers onto a suitable polymeric backbone having reactive functional groups, or (iii) copolymerization of a monomer and a macromonomer. Due to its relative simplicity and because it affords copolymers with well-defined graft lengths, method (iii) was used in the present work. This route has been used previously to prepare poly(S-g-MPEG) [7], poly(MMA-g-MPEG) copolymers [8], and poly(MMA/EHA-g-MPEG) ternary graft copolymers [1]. The success of this method depends on the reactivity of the macromonomer in copolymerization with the comonomer. Steric constraints may reduce the accessibility and reactivity of the macromonomer double bond compared to that of the lower molecular weight comonomer. Thus, longer sequences of BMA (or MMA) will be built

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up during propagation with the MPEG moieties spaced at longer intervals along the chain than expected for a statistical copolymer. As polymerization proceeds and the more reactive BMA monomer is used up, there will be a greater probability of MPEG being incorporated, so that copolymers formed later will be richer in MPEG. In order to help reduce such compositional heterogeneity a monomer feed method was used in the current work. The preparation of both poly(butyl methacrylate-gXmethoxypoly(ethylene glycol)) (poly(BMA-g-XMPEG)) and poly(methyl methacrylate-g-Xmethoxypoly(ethylene glycol)) (poly(MMA-g-XMPEG)) graft copolymers (where X refers to the average wt% of MPEG350MA in the copolymer) was carried out using the following general procedure. AIBN (0.2–4.0 wt%, based on the mass of solvent) was dissolved in a mixture of ethanol and water (50/50 w/w) in a 1 l, four-neck, round-bottomed flask equipped with dropping funnel and reflux condenser. The apparatus was immersed in a silicone oil bath maintained at 80 ◦ C. The solution was then sparged with nitrogen for 20 min, under reflux, before a feed mixture containing BMA (or MMA), MPEG350MA, AIBN and a small amount of ethanol was introduced into the reaction flask at a constant flow rate of 1 ml min−1 . For all of the polymerizations the mole ratio of monomer to initiator in the feed was fixed at 334. The addition of the monomer and macromonomer typically took 30 min and the flow of nitrogen gas was maintained over the surface of the solution throughout the reaction. Polymerization was allowed to proceed to completion overnight. Afterward, the product was extensively dialyzed against Milli-Q water. For the lower molecular copolymers, dialysis was carried out above the cloud-point temperature of the copolymer (ca. 60 ◦ C), but beneath that of the MPEG macromonomer (ca. 95 ◦ C). Following dialysis, the solvent was removed through rotary evaporation and the products dried over phosphorus pentoxide. 2.3. Preparation of micellar solutions Micellar solutions were prepared by direct dissolution of the graft copolymers in water. After heating the dispersions to 60 ◦ C for 1 h they were left for 24 h to equilibrate on a mechanical tumbler. 2.4. Characterization of the graft copolymers and micelles 2.4.1. Small angle neutron scattering (SANS) SANS measurements were performed on the time-offlight diffractometer LOQ using the ISIS pulsed neutron source, CLRC Rutherford Appleton Laboratory, UK. The samples were studied in 2-mm pathlength Hellma cells fitted with Teflon stoppers and sealed with Teflon tape. The SANS experiments were carried out at 25 ◦ C, at a sample to detector distance of 4.1 m, using neutrons of wavelength 0.22– 1.0 nm covering a continuous Q-range of 0.08–2.5 nm−1 .

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The scattering patterns were isotropic and were radially integrated over all azimuthal angles. Scattering intensities were placed on an absolute scale by using the scattering from a well-characterized, partially deuterated polystyrene blend as a calibration sample. The scattering intensity was corrected for detector efficiency, sample transmission, and solvent scattering using the Colette data reduction program [9]. The calculated scattering length densities of the copolymer, D2 O, H2 O, poly(BMA), and MPEG350MA were: +0.704 × 1010 , +6.38 × 1010 , −0.56 × 1010 , +0.595 × 1010 , and +0.731 × 1010 cm−2 , respectively. The experiments were performed in a D2 O/H2 O mixture of scattering length density +4.37 × 1010 cm−2 . Although pure D2 O would have given a better signal, this was chosen to suit the characterization of the latex nanoparticles prepared subsequently using the micelles as templates [4]. An experimentally measured value of 1.13 g cm−3 was used for the bulk density of the copolymer in the calculation of the scattering length density. In a neutron scattering experiment, the differential scattering cross-section (∂Σ/∂Ω)(Q) for a simple uniform particle varies with the scattering vector, Q = (4π/λ) sin(θ/2) according to the expression: ∂Σ (Q) = NP P (Q)S(Q) + B, ∂Ω

(1)

where P (Q) = (ρ)2 V 2 f 2 (Q) is the particle form factor, S(Q) is the interparticle structure factor, NP is the number of scattering centers, V is the volume of one scattering center, ρ = ρparticle − ρsolvent is the neutron scattering length density difference and B is the flat incoherent background signal. The form factor P (Q) contains information on the particle size and shape whilst S(Q) accounts for interactions between different particles. For a dilute, weakly interacting system S(Q) tends to unity and may be ignored. Fits to the SANS data were performed using the FISH program and considering different analytical models for the scattering patterns [10]. This program uses least squares and Marquardt global minimization algorithms to fit the data. After preliminary fits to a series of models a polydisperse core/shell model was selected. In such a case P (Q) contains the terms for both the core and shell and is numerically integrated over a particle size distribution N(R), here applied to the outer radius RT ,
P (Q) = (ρcore − ρshell)VC f (Q, RC ) 2 + (ρshell − ρsolv )VT f (Q, RT ) , (2) where ρcore , ρshell , and ρsolv are the scattering length densities of the core, shell, and D2 O/H2 O water mixture, respectively, and VC and VT are the volumes of the core (radius RC = Y RT ) and the whole particle. For spheres,   3 sin(QR) − QR cos(QR) f (Q, R) = (3) . (QR)3 For the fitting routine, the core was assumed to contain no water but the corona to be swollen with water, with a

uniform scattering density for each so that the average scattering length density of the “wet” shell (ρshell‘wet’ ) is given by ρshell‘wet’ = φsolv,shellρsolv + (1 − φsolv,shell)ρMPEG350MA, (4) where ρMPEG350MA denotes the scattering length density of pure MPEG350MA and φsolv,shell denotes the volume fraction of water in the shell. Different values for φsolv,shell were tested to see which would give the best fit to the experimental data. The Shultz particle size distribution (which is similar to a lognormal), is defined in terms of the polydis (where σ is the standard deviation and R the persity, σ/R average particle size). For a given degree of hydration a parameter, Y , was used to constrain the inner core radius (RC ) to the outer shell radius (RT ), during the integration over the core size distribution, such that RC = Y ∗ RT . The flat background was suitably adjusted; the incoherent and multiple scattering contributions were not accounted for upon subtracting the solvent scattering. The inclusion of a structure factor term was found to be necessary in order to obtain good correlation with the experimental data at small Q for the highest concentration sample (5 wt%). An effective interaction was included using the hard sphere S(Q) according to the Percus–Yevick approximation [11]. 2.4.2. Dynamic light scattering (DLS) The hydrodynamic radius of the micelles, Rh , was determined using DLS. Detailed accounts of DLS theory are given elsewhere [12]. Samples were filtered with an appropriate grade Whatman Anotop filter prior to measurement. Measurements were made on two different instruments, a Malvern 4700 apparatus (µGreenSLM laser, 125 mW, 532 nm) and a Brookhaven Zeta Plus apparatus (He–Ne laser, 5 mW, 632.8 nm) at a fixed scattering angle of 90◦ . Both instruments lead to a z-average (intensity-average) hydrodynamic micelle radius, which is evaluated from the diffusion coefficient, D0 , using the Stokes–Einstein equation, Rh =

kT , D0 η0 6π

(5)

where k is the Boltzmann constant, T is the absolute temperature, and η0 is the viscosity of the medium. For systems adhering to a closed association model the z-average hydrodynamic radius can be expressed as [13]  −1  [Wu Mu (Rh−1 )u + Wm Mm (Rh−1 )m ] , Rh z = [Wu Mu + Wm Mm ]

(6)

where Wu and Wm denote the weight fractions of monomers and micelles, respectively, Mu and Mm are the molecular weights of monomers and micelles, respectively, and (Rh )u and (Rh )m are the hydrodynamic radii of the monomers and the micelles, respectively. Thus, the hydrodynamic radius that is measured will be very close to the micellar radius even if monomers are present because Mm is usually much larger than Mu . In addition to Rh , DLS also gives a polydispersity

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index, µ2 /(Γ¯ )2z , where µ2 is the second moment about the intensity-weighted mean relaxation rate (Γ¯ )2z . The value of µ2 /(Γ¯ )2z provides an indication of the micelle size distribution. Values in excess of 0.2 are generally regarded as being indicative of broad distributions; samples having polydispersity index less than 0.1 are regarded as being monodisperse. 2.4.3. Gel permeation chromatography (GPC) GPC analyses were performed at ICI Paints (Slough, Berks, UK) using two 30 cm PL Gel Mix D GPC columns. The samples were prepared at a concentration of about 1 mg ml−1 in THF (flow rate 1.0 ml min−1 , run time 25 min, 200-µl injection volume). Relative molecular weights were obtained, based on a series of linear polystyrene standards ranging from 580 to 380,000 Da. 2.4.4. Osmotic pressure Osmotic pressure measurements were made with a Gonotec Osmomat 090-SA membrane osmometer using THF as solvent. A membrane with a molecular weight cutoff of 10,000 g mol−1 was used.

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from Nagg =

c − CMC , [M]

(8)

where c is the total polymer molar concentration and CMC is the critical micelle concentration. For this method to be applicable the distribution of probe and quencher over the micelles must be Poisson in form and the quenching in a micelle must be much faster than the fluorescence decay. A third criterion is that both the fluorescent molecule and the quencher molecules must be strongly associated with the micelles and reside in the micelles for a period of time longer than the unquenched lifetime of the fluorescent probe. 2.4.7. Cloud points Cloud points for the aqueous copolymer solutions were obtained either visually or using UV spectrophotometry by heating the solutions with stirring until they became turbid and then recording the temperature at which the solutions became clear again.

3. Results and discussion 2.4.5. Nuclear magnetic resonance (NMR) spectroscopy 1 H NMR spectra were acquired at 23 ◦ C using a Jeol Lambda 300 MHz FT-NMR spectrometer. Tetramethylsilane was used as an internal reference and deuterated chloroform as solvent. The free induction decay spectra were Fourier transformed and resonances of interest integrated using the software Jeol SpecNMR (v. 1.1.4). 2.4.6. Fluorescence Static fluorescence measurements were carried out using a Perkin–Elmer 3000 fluorescence spectrophotometer. Pyrene was used as a fluorescent probe (5 × 10−7 M). The emission spectrum of pyrene was scanned between 350 and 500 nm using an excitation wavelength of 335 nm (excitation slit 15 nm bandwidth, emission slit 2.5 nm bandwidth). Cetylpyridinium bromide was used as a fluorescent quencher in the concentration range 1 × 10−5 –1 × 10−4 M. Dissolved oxygen, being paramagnetic and a strong quencher of fluorescence, was removed from the samples by purging with nitrogen. In static fluorescence quenching, the aggregation number of the microdomains is determined from the decrease of the probe fluorescence as a function of the concentration of an added fluorescence quencher [Q] [14]. The fluorescence intensity, I , in the presence of the quencher is given by   [Q] I = I0 exp − (7) , [M] where I0 is the fluorescence intensity in the absence of quencher and [M] is the concentration of micelles. The latter is obtained directly from the slope of a plot of ln(IQ /I0 ) = f ([Q]). The aggregation number, Nagg may be calculated

3.1. Copolymer synthesis For the formation of nanosize latex particles small micelles are needed [4]. Theoretical studies have shown that for amphiphilic block copolymers the size of the micelle depends on the molecular weight of the copolymer [15,16]. Thus, graft copolymers with different molecular weights and compositions were prepared. The results are listed in Table 1. In the case of the poly(MMA-g-MPEG) graft copolymers (copolymers 15–17, Table 1), there was an increase in molecular weight upon increasing the concentration of MMA in the reaction mixture. Similar increases in molecular weight with MMA concentration have been observed for reactions using 2-butanol as the solvent and was attributed to the higher probability of reaction between the growing PMMA chains and MMA monomer [8]. The use of the transfer agent, isobutylmercaptan (1–3%, based on the weight of monomers), had a negligible effect on the molecular weight of the copolymers (copolymers 18–20). As may be seen from Fig. 1, a much greater change in molecular weight was observed upon increasing the concentration of initiator (AIBN). The decrease in molecular weight with increasing initiator concentration can be explained by the increase in the number of primary radicals for a given concentration of monomer. For radical chain polymerizations, where bimolecular termination between propagating radicals applies, the molecular weight is inversely dependent on the square root of the initiator concentration (i.e., 1/[AIBN]0.5) at low conversions [17]. From Fig. 1 a higher exponent of −0.70 was obtained, which implies that primary termination, involving the reaction of a propagating

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Table 1 Properties of graft copolymers Copolymer Poly(BMA) 1 Poly(BMA-g-20MPEG) 2 Poly(BMA-g-40MPEG) 3 Poly(BMA-g-50MPEG) 4 Poly(BMA-g-60MPEG) 5 Poly(BMA-g-70MPEG) 6 Poly(BMA-g-80MPEG) 7 Poly(BMA-g-80MPEG) 8 Poly(BMA-g-80MPEG) 9 Poly(BMA-g-80MPEG) 10 Poly(BMA-g-90MPEG) 11 Poly(BMA-g-90MPEG) 12 Poly(BMA-g-90MPEG) 13 Poly(MPEG) 14 Poly(MMA-g-80MPEG)15 Poly(MMA-g-80MPEG)16 Poly(MMA-g-70MPEG)17 Poly(BMA-g-80MPEG) 18g Poly(BMA-g-80MPEG) 19g Poly(BMA-g-80MPEG) 20g Poly(BMA-g-80MPEG) 21 Poly(BMA-g-80MPEG) 22 Poly(BMA-g-80MPEG) 23 Poly(BMA-g-80MPEG) 24

[Initiator]a /temp. (wt%/ ◦ C)

MPEG (wt%) (expect)

MPEGb (wt%) (NMR)

Mw c (g mol−1 )

Mw /Mn e

HLBG

BMA units

PEG branches

0.6, 80 0.6, 80 0.6, 80 0.6, 80 0.6, 80 0.6, 80 0.6, 80 1.3, 80 2.0, 80 0.6, 100 0.6, 80 0.6, 80 0.6, 80 0.6, 80 0.4, 80 0.12, 80 0.12, 80 0.6, 80 0.6, 80 0.6, 80 1.0, 80 2.0, 80 3.0, 80 4.0, 80

0 20 40 50 60 70 80 80 80 80 90 90 90 100 82 82 70 80 80 80 80 80 80 80

0 – 41.6 45.0 55.6 68.2 77.6 – – – 86.3 87.3 – 100 80 78 81 80 82 80 77 76 76 68

10,700 6,100 16,000 20,000 16,500 15,900 14,500 10,700 8,000 38,000 15,300 15,500 24,800 17,600 11,400 19,000 23,000 16,600 13,400d 15,500d 13,300d 7,000d 4,900d 4,400d

2.17 2.35 1.94 1.99 1.87 1.84 1.80 1.81 4.75 2.08 1.70 1.69 1.97 2.12 1.6 1.7 1.8 2.1 – – – – – –

– – 6 7 8 10 11.5 – – – 13 13

34 14f 34 38 27 19 12 8f 2f 25f 6 6 8f – 23 38 46 23 17 22 22 12 8 10

– 1f 8 11 12 14 15 11f 3f 35f 19 20 27f 20 22 37 45 32 27 30 25 13 9 7

15 12 12 10.5 12 12 12 11.5 11.5 11.5 10

Poly(BMA-g-XMPEG) and poly(MMA-g-XMPEG) graft copolymers (where X refers to the average wt% of MPEG350MA in the copolymer). a wt% based on mass of solvent. b wt% determined using 1 H NMR after dialysis. c GPC molecular weights correspond to the dialyzed products (molecular weights were calculated relative to a series of linear polystyrene standards ranging from 580 to 380,000 Da). d GPC molecular weights were measured using the crude products (hence, polydispersities not shown). e Polydispersity according to GPC after dialysis. f Values estimated from the expected amount of MPEG. g Chain transfer agent, isobutylmercaptan added (copolymer 18: 1%, copolymer 19: 2%, copolymer 20: 3%, based on weight of monomers).

Fig. 1. Dependence of molecular weight of poly(BMA-g-80MPEG) graft copolymers (7–9, 18, 21–24) on initiator (AIBN) concentration. Gradient = −0.70.

radical with a primary radical [18], is significant in the initiator concentration range used here. A primary termination mechanism will lead to the presence of free cyanoisopropyl radicals at the end of the reaction, especially at higher ini-

tiator concentrations, thus rationalizing the appearance of long, needle-shaped crystals of tetramethylsuccinodinitrile (as determined from 1 H NMR, IR, and mass spectra investigations) in solution upon storage of the poly(BMAg-80MPEG) copolymers (21–24) at low temperatures (ca. 4 ◦ C). This toxic compound is usually formed as a byproduct when cyanoisopropyl radicals recombine in a process referred to as the “cage-reaction” [19]. Previous studies found that uncontrolled cross-linking and gel formation occurred when toluene or 2-butanol were used as solvents. This behavior was attributed either to the high probability of chain transfer to polymer [1] or the presence of bifunctional PEG left over from the preparation of the macromonomer [8]. In order to reduce the likelihood of gel formation, the polymerization reactions in the current work were carried out at low concentrations in a solvent for which there is less likelihood of hydrogen atom abstraction. On the one occasion where a gel was observed (poly(BMA-g-20MPEG) 2—number in bold denotes the position in Table 1; number in brackets indicates the weight percentage macromonomer in the copolymer), the gel did not appear to be cross-linked and dissolved easily in acetone.

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The formation of a physical gel with on increasing amount of BMA in the copolymer may be attributed to the greater hydrophobicity of this copolymer and its poorer solubility in ethanol/water solvent mixtures. 3.2. Copolymer purification Attempts to purify the copolymers from the unreacted macromonomer by precipitation methods were unsuccessful due to the similar solubilities of the graft copolymer and macromonomer in most solvents. As an alternative means of purification, unreacted monomer was removed from the asmade copolymer solutions by extensive dialysis against pure water. The effectiveness of this method was confirmed by comparing the GPC traces of the copolymers before and after dialysis and by examining the 1 H NMR spectrum of the dialyzed products in the region δ = 5–6.5 ppm for signs of monomer/macromonomer double bonds. For those copolymers with molecular weights beneath the cut-off point of the dialysis membrane, the difference in the cloud-point temperatures of the graft copolymer and the macromonomer was exploited as a way of separation. For all of the polymerizations the copolymer yields, based on analysis of their dry weights after purification, were high (>90%), indicative of high conversions. The as-made graft copolymers prepared using MPEG 350MA were pale yellow, viscous/waxy oils, in contrast to copolymers incorporating MPEG2000, which tend to be crystalline, white solids [2]. Gramain and Frere observed that side-chain crystallinity is only seen in polymers containing PEG methacrylates with more than 6–7 ethylene oxide (EO) units (i.e., molecular weights >350 g mol−1 ) [20]. This is because oligomers with less than 6–7 EO units cannot adopt the 7/2 helix in a monoclinic lattice [21]. Moreover, the crystallization of shorter PEG side-chains (e.g., mwt 350–750 g mol−1 ) is prevented by their better compatibility with the methacrylic backbone. 3.3. Copolymer characterization Representative 1 H NMR spectra of the poly(BMA-gMPEG) copolymers are shown in Fig. 2. The spectra consist of a series of resonances in the regions δ = 1–2 ppm and δ = 3.5–4.2 ppm. The spectral assignments in Fig. 3 were made on the basis of these spectra, together with assignments made elsewhere in the literature for similar copolymers [1,7]. By integrating the resonances located at δ = 3.4 ppm and between δ = 3.5 ppm and 3.8 ppm the average number of ethylene oxide (EO) units per MPEG unit was calculated. The amount of BMA and PEG incorporated into the copolymer, meanwhile, was evaluated from the integrated signals corresponding to the methylene protons adjacent to the carboxyl group (–COOCH2 –) in the PEG side chain (δ = 4.1 ppm) and the equivalent methylene protons on the butyl chain (δ = 3.9 ppm) (for methyl methacrylate the resonance corresponding to –COOCH3 is located at

Fig. 2. Representative 1 H NMR spectra: poly(MPEG) 14; poly(BMA-g70MPEG) 6; poly(BMA-g-50MPEG) 4; poly(BMA-g-20MPEG) 2; and poly(BMA) 1 in CDCl3 (bottom to top).

Fig. 3. Spectral assignments for poly(BMA-g-MPEG) copolymers (δ ppm). The number in brackets corresponds to the methylene protons nearest to the carboxyl group.

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Table 2 Number-average molecular weights of poly(BMA-g-MPEG) copolymers in THF according to GPC and membrane osmometry Copolymer

Mn (g mol−1 ) [GPC]

Mn (g mol−1 ) [osmometry]

5 12

8800 9200

23870 22950

GPC analyses performed at ICI Paints (Slough, Berks, UK) using two 30 cm PL Gel Mix D GPC columns. Samples were made up at a concentration of about 1 mg ml−1 in THF (flow rate, 1.0 ml min−1 ; run time, 25 min; 200 µl injection volume). Molecular weights were calculated relative to a series of linear polystyrene standards ranging from 580 to 380,000 Da.

δ = 3.6 ppm). As can be seen from Fig. 2, the intensity of the signal corresponding to BMA increases on reducing the amount of macromonomer used in the synthesis. 1 H NMR may only provide an indication of the average composition of the graft copolymers and cannot be used to determine the compositional heterogeneity or the monomer sequence distribution. Thus, when copolymer compositions (in wt% MPEG) are related to bulk properties in the current study, it must be remembered that in each sample there will be a range of copolymers with different molecular weights, compositions and architectures. Following the definition of Griffin [22], the hydrophile– lipophile balance (HLB) values of the copolymers investigated were calculated to be between 6 and 15. Table 2 shows the number-average molecular weights determined for two copolymers (copolymers 5 and 12) using membrane osmometry and GPC. The former values were found to be approximately twice as large as the latter. However, previous studies involving amphiphilic copolymers have shown that care must be exercised when correlating molecular weights with GPC elution volumes for polymers that have different chemical compositions [3,23–26]. The polydispersity values of the copolymers in this work were rather high (Mw /Mn ∼ 1.6–4), indicative of the range of competing termination and chain transfer reactions that are possible in free-radical polymerizations. 3.4. Aqueous solution behavior Aqueous solutions of the graft copolymers were found to undergo phase separation upon heating. For the poly(BMAg-MPEG) copolymers, the cloud-point of a 1 wt% solution decreased with the overall molecular weight (Table 3), in agreement with the behavior shown by PEO–PPO–PEO Table 3 Change in cloud point temperature (CPT) with molecular weight of poly(BMA-g-MPEG) copolymers Copolymer

Mw (g mol−1 ) [GPC]

CPT (±0.2 ◦ C)

18 21 22 23 24

16,600 13,300 7,000 4,900 4,400

53.6 52.3 51.1 48.1 45.8

Determinations were made with 1 wt% solutions using UV spectroscopy.

block copolymers, e.g., the Pluronic series [27]. On the basis of entropy, it might be expected that smaller molecules would have a greater tendency to remain in solution, compared to larger ones, and that the CPT would increase with decreasing molecular weight. An explanation for the observed behavior is the slight decrease in the proportion of PEG in the copolymer on descending Table 1 from copolymer 18 (80% PEG) to copolymer 24 (68% PEG). Usually it is found that the cloud point temperature increases with the PEO content of the copolymer [1,27]. The temperature at which phase separation occurred also decreased with increasing polymer concentration. Indeed, at solution concentrations above 2 wt%, the copolymer with the lowest molecular weight (poly(BMA-g-80MPEG) 24) gave turbid solutions at room temperature and displayed upper critical solution temperature (UCST) behavior. For the preparation of the polystyrene nanoparticles, as described in the accompanying paper [4], the copolymer concentration must be above the CMC so that micelles exist. Spectroscopic techniques, based on either the emission or absorption of light by a suitable molecular probe, are frequently used to determine CMC values of amphiphilic copolymers [28]. Pyrene was selected in the current work as a probe because it is sparingly soluble in water (6 × 10−7 mol l−1 ) and readily partitions into the hydrophobic interiors of aggregates. Additionally, its emission spectrum displays considerable fine structure, which varies with small changes in the polarity of the probe’s environment (the “Ham effect”) [29]. Since pyrene can also be used at very low concentrations (5 × 10−7 M), the effect of the probe on the solution behavior of the amphiphile is thereby limited. The emission spectrum of pyrene exhibits five major vibronic bands. The third band, at 385 nm, shows the largest perturbation in intensity relative to the first band (located at 374 nm) as the probe’s environment changes from molecularly dispersed copolymer to aggregated micelles. Therefore, the change in the ratio of intensities of these two bands (referred to as the III/I ratio) may be followed to locate the CMC. Figure 4 shows how the III/I ratio varies with the logarithm of the polymer concentration for three poly(BMA-gMPEG) copolymers (copolymers 5, 12, and 18) with similar molecular weights but different average compositions. Over the concentration range 1 × 10−5 –1 × 10−3 g l−1 , where monomers are believed to exist, only a very gradual increase in the III/I ratio is noticeable, while in the region between 0.001 and 0.05 g l−1 , the III/I ratio rises more steeply as a result of solubilization of pyrene in the micelle core. It is noticeable that the monomer-to-micelle transition is considerably broader than that of typical low molecular weight, conventional nonionic surfactants, such as alkyl ethylene oxides. Such molecules usually exhibit very sharp monomerto-micelle transitions whose breadth increases with a decrease in the aggregation number. Amphiphilic copolymers, on the other hand, tend to exhibit broad transitions, regardless of the actual value of the aggregation number. This difference in behavior of the two types of amphiphile is usually

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Fig. 4. Variation in III/I ratio with graft copolymer concentration. Poly(BMA-g-90MPEG) 12 (downward triangles), poly(BMA-g-80MPEG) 18 (circles), poly(BMA-g-60MPEG) 5 (upward triangles). [Pyrene] = 5 × 10−7 M.

Fig. 5. Variation in III/I ratio with graft copolymer concentration. Poly(MMA-g-80MPEG) 15 (squares), poly(MMA-g-80MPEG) 16 (circles), poly(MMA-g-70MPEG) 17 (upward triangles). [Pyrene] = 5 × 10−7 M.

attributed to copolymer polydispersity, compositional heterogeneity, and the presence of impurities [27]. The poly(MMA-g-MPEG) copolymers (copolymers 15–17) prepared in this study exhibited similar behavior to the poly(BMA-g-MPEG) copolymers (Fig. 5). However, the midpoints of the monomer-to-micelle transitions (assumed to correspond to the CMC) were slightly lower. For both types of copolymer, polymers with larger, more hydrophobic domains form aggregates at lower concentrations. Similar behavior has been observed for PEO–PPO–PEO block copolymers [28], PEG/PLA block copolymers [30] and comb-shaped amphiphilic graft copolymers based on MPEG [1]. The concentration ranges where the monomerto-micellar transition occurs obtained in this work are commensurate with values reported in the literature for other amphiphilic copolymers [30–32]. Further evidence of copolymer aggregation and micelle formation of the graft copolymers prepared in this work in aqueous solution was derived from the variation

555

Fig. 6. 1 H NMR spectra of poly(BMA-g-80MPEG) 18 showing line broadening of backbone resonances (δ ≈ 0.9–1.7 and methylene protons of BMA at δ 3.8–3.9) in D2 O (top). The 1 H NMR spectrum is shown in CDCl3 for comparison (bottom). The peaks located at approximately 4.8 ppm (top) and 7.5 ppm (bottom) are from the solvents.

in the reduced viscosity (ηsp /c) with increasing copolymer concentration: a significant decrease was observed, ∼5 × 10−4 g ml−1 , but a gradual increase in ηsp /c at higher polymer concentrations. Since the reduced viscosity is proportional to the ratio of volume/mass of the solute molecules, the former observation is indicative of aggregation (i.e., that the mass of the micelle increases faster than the volume), while the latter observation may be explained by enhanced micellar interactions due to the closer proximity of the micelles to each other at higher concentrations. Similar behavior has been reported for poly(MMA-g-PEO) graft copolymers in toluene [33]. A linear least squares analysis of the reduced viscosity data for copolymer 18 (not shown), ignoring the initial decrease in ηsp /c, yielded a value of 5.5 ml g−1 for the intrinsic viscosity [η]. An indication as to the morphology of the copolymer aggregates in water was derived from comparisons of the 1 H NMR spectra of the copolymers in different solvents [34,35]. Representative spectra for poly(BMA-g80MPEG) 18 are shown in Fig. 6. In CDCl3 , the resonances corresponding to the protons of the grafts and the backbone are visible since CDCl3 is a good solvent for both components. In deuterated water, however, the signals corresponding to the backbone are seen to broaden. This is due to extensive magnetic dipole interactions between the BMA nuclei on account of their close confinement in the micelle core. 3.5. Micelle properties The variation in micelle size with the average composition of the poly(BMA-g-MPEG) copolymers (copolymers 3–7, 9–12, 14, 18–23) is shown in Fig. 7. The micelle size was found to be independent of the polymer concentration over the range investigated (0.1 to 3.0 wt%). Such

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Fig. 7. Variation in size of poly(BMA-g-MPEG) micelles with wt% MPEG350 in the copolymer. Data obtained using 1 wt% aqueous copolymer solutions (a line is drawn in as a guide for the eye).

concentration independence is usually a sign that a singlesized micellar species exists and that the association may be best represented by a closed association model, rather than a multiple association process. The latter is characterized by aggregate growth and high polydispersity indices. In this study, the polydispersity values of the micelles appeared to be independent of the nature of the copolymer with values typically in the range 0.1–0.25. Eckert and Webber observed that while napthalene-tagged copolymers based on polystyrene-alt-maleic anhydride-graft-poly(ethylene oxide) were typically polydisperse ((Mw /Mn )DLS ∼ 1.5), the micelles derived from the graft polymers were reasonably monodisperse [5], suggesting that, when amphiphilic polymers self-assemble into micelles, they incorporate differentmolecular-weight polymers as required to achieve the most stable shape and density of surface amphiphiles. In contrast to the pronounced variation in size seen with composition, no correlation between copolymer micelle size and molecular weight was observed in this study. Within the set of copolymers having nominally 80% MPEG content, the size varies widely (Fig. 7). It is noticeable from Fig. 7 that above 80 wt% MPEG the aggregate size appears to plateau at around 10 nm and the change in size becomes less dramatic with composition due to an increase in the density of PEG branches, which will tend to oppose micelle formation. Pluronics with similar PEO contents (70–80 wt% PEO) and molecular weights (ca. 10,000 g mol−1 ) as the copolymers in Table 1 are reported to have monomer sizes typically of 1 nm while the micelles are thought to have dimensions on the order of 4– 10 nm [27]. Therefore, the “scattering bodies” in the region around 80 wt% MPEG must represent micelles rather than molecularly dispersed monomer. For one of the copolymers containing 80 wt% MPEG, (poly(BMA-g-80MPEG) 18), the weight-average micellar aggregation number, Nagg , was obtained by dividing the weight-average molecular weight of the micelle, Mw,p , derived from a Zimm plot of neutron scattering data (see

Fig. 8. Zimm plot to SANS data for aqueous solutions of poly(BMA-g-80MPEG) 18 showing line for Q = 0: 1 wt% (circles), 2 wt% (downward triangles), 5 wt% (upward triangles). k1 = (ρ)2 /NA δ 2 , where NA is Avogadro’s number, ρ is the difference in the neutron scattering lengths, and δ is the density of the copolymer. c represents concentration; α is a constant that has been added to help separate the data.

Fig. 8) [36], by the known weight-average monomeric molecular weight. For simplicity, the difference in contrast between the PBMA and PMPEG blocks of the copolymer was ignored in the analysis of the data, and the polymer was assumed to be “dry.” No corrections were made to the concentrations used in the Zimm plot since the monomer to micellar transition for this polymer was very low (∼1 × 10−5 g cm−3 ). From the intercept with the y-axis Mw,p was determined to be 59,600 g mol−1 . The molecular weight of the monomer, meanwhile, according to GPC was 16,600 g mol−1 , thus giving a value of approximately 3–4 for the aggregation number of the poly(BMA-g-80MPEG) 18 micelles. This value was in good agreement with the value obtained (Nagg = 3) by the simple method of Turro and Yekta. There the decrease in fluorescence intensity via quenching of a luminescent probe by a known amount of quencher molecules is monitored (results not shown) [14]. Allowing, say, 3, 4, or 5 water molecules per ethylene oxide moiety in the MPEG shell around the micelles changes the contrast and concentration in the Zimm plot in opposite directions so that aggregation numbers are around 3.8, that is, similar to the value of 3.6 for a “dry” aggregate. The second virial coefficient, A2 determined from the slope of the Zimm plot (Q = 0) in Fig. 9, was calculated to be +1.62 × 10−4 cm3 mol g−2 , indicating that a fairly large repulsive interaction exists between the micelles. In contrast, using static light scattering A2 is usually of the order of 10−5 –10−6 cm3 mol g−2 for aqueous solutions of amphiphilic copolymers [37]. The higher value obtained using neutron scattering reflects the enhanced interactions between scattering bodies at the higher particle concentrations typically used for neutron scattering experiments. The suggestion from combination of fluorescence and DLS data that smaller aggregates with low aggregation numbers form as the ratio of MPEG: BMA increases agrees with findings for polystyrene-alt-maleic anhydride-

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of 3.9 nm was found for Rg . This compares with an Rh value of 5.0 nm and a viscometric radius, Rv of 3.7 nm, which was obtained from the expression   3[η]Mw,p 1/3 Rv = (9) , 10NA π

Fig. 9. Representative Guinier plot to low-Q SANS data for poly(BMA-g80MPEG) 18 (2 wt% aqueous solution).

g-poly(ethylene glycol) copolymers. For these copolymers Nagg was found to vary inversely with the grafting density −3 (where NPEO according to the relationship Nagg ∼ NPEO is the number of PEO grafts per chain) when the degree of polymerization of the core block was constant [5]. As mentioned by the authors of this reference, for colloidal stabilization it is reasonable to assume that one of the main criterion for stabilization is the optimal area for each PEO chain at the micelle surface. Thus, when the degree of polymerization of the core block is constant, the aggregation number of the micelle must also vary inversely with the area per PEO coil on the micelle surface (SPEO ) according to the −3 relationship Nagg ∼ SPEO . The shape and structure of the micelles formed by poly(BMA-g-80MPEG) 18 were further studied as these were used as templates for nanoparticle preparation, as described in the accompanying paper [4]. The radius of gyration of the poly(BMA-g-80MPEG) 18 micelles was measured as a function of concentration by constructing Guinier plots of the SANS data at low Q [38], as exemplified in Fig. 9. Using a linear least-squares fitting procedure to fit the data in the region QRg ≈ 1 (Q2 = 0.003 Å−2 ) a value

where [η] is the intrinsic viscosity obtained earlier. In the Guinier approximation the value of Rg obtained sensitively depends on the range of Q2 that one chooses to fit. One necessary condition is that QRg must be less than unity. However, moderate polydispersity reduces the curvature of the plot so that this constraint can be relaxed, as can be seen from the inset of Fig. 9 [39]. From the characteristic radii Rg and Rh it is possible to obtain insights into the inner structure of the micelles. The ratio Rg /Rh is reported to be 0.775 for homogeneous hard spheres and greater than 2 for monodisperse rodlike polymers. For elliptical structures, meanwhile, Rg /Rh depends strongly on the axial ratio. For poly(BMAg-80MPEG) 18 a value of Rg /Rh of 0.78 was calculated, which is slightly above the hard-sphere value. In order to further analyze the structure of the micelles the SANS data for poly(BMA-g-80MPEG) 18 were fitted to a variety of analytical models. From amongst the models a polydisperse core/shell model for a sphere was chosen. Hard-sphere structure was included in the model to account for the apparent influence of interparticle scattering. Preliminary fits to the neutron scattering data resulted in much lower copolymer volume fractions than expected. This may result if PEO is able to coordinate water, thereby raising the scattering length density of the PEO segments (ρMPEG350MA) relative to the core block whilst reducing the contrast between the hydrated shell and the solvent [40]. According to King et al., values for the degree of hydration (W ) may vary from about one to seven water molecules per ethylene oxide segment [41]. The model was, therefore, modified to include a separate “wet” shell, constrained to give the expected core/shell volume ratio as outlined in the experimental section. Some of the results obtained from the fitting procedure are tabulated in Table 4 while the resultant fits are shown in Fig. 10. The best agreement to the absolute intensities of the experimental data was obtained when the micelle shell contained 65–70% water corresponding to four

Table 4 Typical values obtained from SANS fit to polydisperse core/shell spheres (FIT) compared to expected values (EXP) for poly(BMA-g-80MPEG) 18 [Copolymer 18] (wt%) (EXP)

W water per EO (±1)

φsolv,shell water in shell (%)

ρshell,wet (1010 cm−2 )

Y = RC /RT

T (Å) R outer (FIT)

vol% total (FIT)

vol% total (EXP)

(σ/R) (±0.05)

HS radius (Å) (FIT)

HS S(Q) (vol%) (FIT)

Nagg

1 2 5

4 4.5 5

64.9 67.5 69.8

3.09 3.19 3.27

0.432 0.422 0.413

22.5 21.7 24.5

2.29 4.97 13.43

2.36 5.07 13.56

0.35 0.35 0.26

– – 64.1

– – 2.2

1.8 1.5 1.7

For a best fit at W = 4 water per EO, taking instead W = 3 (58% water) or W = 5 (70% water) moves the fitted volume parameter down or up by about 10% (slightly more than the expected systematic errors in the absolute values of the SANS data and uncertainties in sample composition and bulk density of 1.13 g ml−1 ). Changing the polydispersity from 0.35 to 0.3 or 0.4 moves the volume parameter down or up by around 5%. The mean aggregation number Nagg R T , and the BMA core molecular weight in Table 1. The 5 wt% concentration is estimated from the mean core volume, of radius (1 + (σ/R))(1 + 2(σ/R))Y required an effective interparticle S(Q) for which a hard sphere model was used, though a “softer” interaction might have provided a more realistic S(Q) volume fraction and radius.

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transitions with increasing concentration, in contrast to the sharp transitions usually observed for conventional surfactants. DLS measurements reveal that there is an increase in the size of the micelles with an increase in the hydrophobicity of the copolymers. With decreasing hydrophilicity, interchain hydrophobic interactions increase, thus favoring aggregation. As the proportion of MPEG350 in the copolymer increases and the copolymers become more hydrophilic, micelles with characteristic dimensions of ∼ 10 nm form. For one copolymer (poly(BMA-g-80MPEG) 18) which formed micelles of 10 nm in diameter, it was shown that the micelles behave as hard spheres with highly water-swollen corona. In the second part of this study the use of these micelles as templates for the preparation of polystyrene nanoparticles will be demonstrated [4].

Fig. 10. SANS data for aqueous solutions of poly(BMA-g-80MPEG) 18 at different concentrations: 5 wt% (upward triangles), 2 wt% (downward triangles), and 1 wt% (circles). A core/shell model with ∼ 65% water in the shell was used to fit the data. The 5 wt% required a modest interparticle structure factor shown dashed (×2.0). The inset shows a typical Schultz = 25 Å, σ/R = 0.35. polydispersity for the outer radius of R

or five molecules of water per EO segment. A mean particle T ) of about 2.2 to 2.5 nm was obtained, but with a radius (R fairly large polydispersity of around 35%. The mean aggregation number from these model fits is around 1.5 to 1.8, a value lower than that estimated from the Zimm plot and static fluorescence measurements, but the fluorescence method assumes monodisperse materials while the Zimm method is biased to larger particles. The shape of the SANS data here is not particularly sensitive to the size of the dry core due to VC being small compared to VT in Eq. (2) and the fact that the two components of the polymer have similar neutron scattering length densities. The absolute intensity is however sensitive to the mean composition of the micelles and the shape of the diffraction patterns to their total size. To obtain more detailed information on the size of the core and the nature of the core/shell and shell/solvent interfaces one or other of the polymers would need to be deuterated and use made of solvent contrast variation with a range D2 O/H2 O mixtures. Then the scattering length densities in Eq. (2) may be arranged so that the two terms have opposite signs and a characteristic, hollow shell, interference results.

4. Conclusions Water-soluble, comb-shaped amphiphilic copolymers have been prepared by radical solution polymerization of poly(ethylene oxide) methacrylate macromonomers and methacrylic comonomers in a water/ethanol mixture (50/50 w/w) without the often encountered problem of uncontrolled cross-linking and gel formation. When dissolved in water, the purified copolymers exhibit broad monomer-to-micelle

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