Marianne Gaborieau
email: m.gaborieau@uq.edu.au

Robert Graf
email: graf@mpip-minz.mpg.de

Stefan Kahle
email: kahle@mpip-mainz.mpg.de

Tadeusz Pakula

Hans W Spiess
email: spiess@mpip-mainz.mpg.de

Chain dynamics in poly(n-alkyl acrylates) by solid-state NMR, dielectric and mechanical spectroscopies

Keywords: poly(n-alkyl acrylate), polyacrylate, relaxation, chain dynamics, solid-state nuclear magnetic resonance (NMR), nuclear Overhauser effect (NOE), dipolar filter, dielectric spectroscopy, mechanical spectroscopy

published or not. The documents may come

Introduction

Poly(alkyl acrylates) are of industrial importance, due to their wide use in e.g., pressure-sensitive adhesives (PSAs), paints, and coatings. 1 Most of those products are currently developed by trial-anderror procedures, 2 necessitating synthesis and tests of functional properties for numerous samples. Therefore, it is important to be able to characterize and understand the polyacrylate properties on a molecular level. On a long term, relating these to macroscopic properties (mechanical, adhesive), will allow one to directly design samples for specific applications rather than testing numerous samples.

In the present work, chain dynamics occurring in polyacrylate melts were investigated to gain insight on relaxation processes occurring on the molecular level. Poly(n-alkyl acrylate) homopolymers with various alkyl side chains were chosen as model samples for multi-component poly(alkyl acrylate)-based industrial samples. We have recently developed a solid-state NMR method to quantify relaxation processes in homopolymers in the melt. 3 This method is based on measurements of relative motions of neighboring 1 H nuclei, thus it is able to detect relaxation processes different from that usually detected by dielectric spectroscopy or dynamic mechanical measurements. It is applied to poly(n-alkyl acrylate) homopolymer melts in the present work. The results are compared with dielectric and dynamic mechanical results.

In poly(n-alkyl acrylates), short-and long-chain branching [4][5][6] as well as a local nanostructure 7,8 are present. The relaxation processes in polymers are influenced by long-chain branching, as shown for rheological properties of polyethylene. 9,10 Therefore dielectric spectroscopy and dynamic mechanical analysis data used for comparison with results from the solid-state NMR method were measured on the same samples, rather than only taken from literature. Furthermore, in order to characterize the local nanostructure, X-ray measurements were carried on the same samples.

The general formula of poly(n-alkyl acrylates) is shown in Figure 1. The homopolymers with the following alkyl side chains were synthesized: methyl (PMA, x=1), ethyl (PEA, x=2), butyl (PBA, x=4), hexyl (PHxA, x=6). Free-radical polymerization in solution was chosen to obtain broad molar mass distributions and branching levels similar to industrial samples, and avoid the presence of surfactants.

Experimental section

Materials. 11 Each n-alkyl acrylate (Aldrich, 98 or 99 %, stabilized with hydroquinone or monomethyl ether hydroquinone) was distilled under pressure and stored at -20 °C. It was polymerized as a 4.7 mol.L -1 solution in toluene initiated by 0.5 mol% of AIBN (azo-bis-isobutyronitrile) with respect to the acrylic monomer. The polymerization was carried out at 60 °C under nitrogen for 20 h. The resulting reaction mixture was dissolved in an equal volume of dichloromethane. Then the polymer was precipitated in methanol at a temperature lower than its glass transition temperature (Tg), filtered and finally dried in an oven (60 °C) under vacuum for one night. Please note that alkyl acrylates monomers are toxic, irritant, and can induce sensitization.

The results of the characterization of the poly(n-alkyl acrylates) are given in Table 1. The Tgs were determined using differential scanning calorimetry (DSC) at 10 K•min -1 ; the detected Tg is in accordance with the values found in the literature 1 for each sample (taking into account the influence of the heating rate on the detected Tg). 13 C NMR spectra in solution in CDCl3 were recorded on a Bruker DRX500 at 125.76 MHz, with a relaxation delay of 10 s and 19000 to 21000 transients. 11 All samples are atactic and their branching levels are several percents of the monomeric units. The average molar masses were determined via size exclusion chromatography (SEC) in THF with universal calibration using online refractometer and viscometer; the experimental set-up has been described elsewhere. 11 Since the polyacrylates are branched, their separation by SEC is not complete; thus the newly developed formalism for proper determination of average molar masses of branched samples using hydrodynamic volumes distributions 12 has been used. More details on sample characterization are given in supporting information.

Methods.

Dielectric spectroscopy. A broad band dielectric spectrometer Novocontrol BDS 4000 (based on the high resolution ALPHA Analyzer) was used to measure the dielectric function *()='() -i "() at 10 points per frequency decade in a frequency range of  = 10 -2 -10 7 Hz and a temperature range from -120 to 100°C. The sample capacitor consisted of two parallel gild brass-plates with a diameter of 20 mm, separated by the sample. The plate distance of 0.1 mm was kept constant by small Teflon spacers.

According to linear response theory the dielectric data can be also described by the corresponding complex dielectric modulus M*() where *()M*()=1 with *()='() + i "(). Figure 2 shows the temperature and frequency dependence of the imaginary part of the dielectric modulus of the four polyacrylate samples. For each sample at least three relaxation processes were observed: conductivity contributions and segmental and local relaxations.

Using the nonlinear least-square Levenberg-Marquard method, each relaxation peak (M'') was fitted by the imaginary part M'' of the Havriliak-Negami (HN) funtion M*: 13 ( )

( )      i 1 ) ( * +  + =  M M M (1) and ) sin( ) ( ' ' -      M M  = (2)
with

       2 ) ( /2) cos( ) ( 2 (1 + + = (3) and ( ) ( ) 2) / cos( ) ( 2 / sin tan -      + = A (4)
where =2 is the angular frequency and  the intensity of the process. The maximum loss frequency max for each process was analytically calculated from the fit parameters by 14

             / 1 max 2 / cos ) 1 ( 2 / tan 2 / sin 1 -         - + = (5)
For   max and   max the HN function for "() reduces to power laws with the exponents  and , respectively. To fit the -process, all parameters were free, while for the -processes the asymmetry parameter  was set to 1 (this corresponds to a symmetric peak which is typical for the process). Note, that the HN function is usually used to fit the dielectric function and not for the dielectric modulus. However, the imaginary part of the HN function is also a good choice to describe the imaginary part of the dielectric modulus. For the real part of the modulus, of course, the HN function does not work.

Dynamic mechanical measurements. Dynamic mechanical measurements were performed using the ARES mechanical spectrometer (Rheometric Scientific). Shear deformation was applied under condition of controlled deformation amplitude, always remaining in the range of the linear viscoelastic response of studied samples. Geometry of parallel plates was used with plate diameters of 6 mm. The gap between plates (sample thickness) was about 1 mm. Experiments were performed under dry nitrogen atmosphere. Frequency dependencies of G' and G'' measured within the frequency range 0.1 -100 rads -1 at various temperatures were used to construct master curves representing a broad range frequency dependencies of these quantities. Only shifts along the frequency scale were performed. This procedure provided a temperature dependence of shift factors (log aT vs T). The relaxation times corresponding to the transition to the Newtonian flow range at low frequencies (N) and to the glassy range at high frequencies (s) at the reference temperature have been determined as

  / 1 ) ( = ref T ,
where  is the frequency at which the G' and G'' dependencies cross each other at corresponding frequency ranges. Relaxation times at another temperatures are given by

T ref a T T log ) ( log ) ( log + =   .
Solid-state NMR. Solid-state NMR measurements were carried out on a Bruker DSX300 spectrometer at Larmor frequencies of 300.13 MHz for 1 H and 75.47 MHz for 13 C using commercial 7.5 mm static and 4 mm magic angle spinning (MAS) double resonance probes from Bruker BioSpin GmbH. For the measurements done under static conditions, the actual sample temperature was calibrated using lead nitrate and a few melting points. 11

2D-WISE spectra 15 were recorded under static conditions for PMA, PEA, PBA and PHxA at Tg+70 K using a 7 mm rotor. A 5 µs 90° 1 H pulse was used, followed by a 180° pulse in the middle of indirect time to refocus the chemical shifts, in order to acquire the static 1 H line width in the indirect dimension.

To avoid 1 H spin diffusion, polarization transfer from 1 H to 13 C was accomplished by Lee-Goldburg cross-polarization 16 , with 500 µs contact time. During 13 C acquisition, 1 H continuous wave decoupling at 50 kHz RF nutation frequency was applied. 144 to 176 increments were acquired in the indirect 1 H dimension and 240 to 304 transients were recorded in the direct 13 C dimension with 2 s relaxation delay between consecutive transients.

Lee-Goldburg cross-polarization (CP) MAS spectra were recorded at 3 kHz MAS spinning frequency using 4 mm rotors. In the 1 H channel the RF power level was set to a nutation frequency of 83 kHz (corresponding to a 90° pulse length of 3 µs) for excitation as well as for 1 H continuous wave decoupling during acquisition. A relaxation delay of 3 s was chosen for these experiments. The Lee-Goldburg irradiation 16 for the CP was adjusted on the 1 H nuclei by first calculating the corresponding irradiation offset frequency for the chosen RF power level and then finely adjusting the irradiation power by optimizing the resolution of 13 C multiplets under Lee-Goldburg decoupling conditions. The following series of experiments was conducted: first a simple LG-CP spectrum, second a LG-CP spectrum recorded immediately after a dipolar filter, third a LG-CP spectrum recorded after a dipolar filter with the same experimental settings and a subsequent mixing time. The experiments were conducted on PEA at ca 329 K (ca Tg+70 K) with a CP contact time of 1.5 ms, a filter with 20 µs delay and 4 cycles, a mixing time of 20 ms, and 2560, 5120, 8192 transients for the three spectra, respectively.

The experiments were conducted on PBA at room temperature (ca Tg+70 K) with a CP contact time of 3 ms, a filter with 20 µs delay and 8 cycles, a mixing time of 50 ms, and respectively 3072, 8192, 8192 transients for the three spectra.

The principle of the nuclear Overhauser effect (NOE) experiment with dipolar filter was presented in detail previously. 3 62.5 kHz RF nutation frequency and 5 s recycle delay have been used. The delay between pulses in the dipolar filter has been varied from 10 to 20 ms, and the number n of cycles in the dipolar filter from 1 to 12, depending on the sample and the temperature. For each sample at each temperature, an average of the correlation times obtained with different filter parameters has been calculated. The measurements have been carried out at temperatures ranging from Tg+75 K to Tg+100 K on sample PMA, from Tg+20 K to Tg+100 K on samples PEA, PBA, PHxA.

Results

Dielectric Spectroscopy. The dielectric measurements were done and processed as frequencydependent measurements (see experimental part). Three-dimensional plots of the full datasets are given in supporting information. Figure 2 (right) shows slices of these three-dimensional plots at four frequencies (10 -1 , 10 1 , 10 3 , and 10 5 Hz), which are examples of the temperature dependence of the imaginary part of the dielectric modulus for the different polyacrylate samples. The corresponding relaxation processes are compiled in Figure 2 (left). At a given frequency, the  process occurs at lower temperature with increasing side chain length, as expected from the decreasing glass transition temperatures. Similar results were observed in the homologous series of the poly(n-alkyl methacrylates). 17 The  process is only observed in the butyl and hexyl members. In the methyl and ethyl members, the intensity seems too small to detect this process (note that in a commercial poly(methyl acrylate) two local relaxations were previously observed 18 ). For the butyl and hexyl members, the  process shifts to lower frequencies with increasing side chain length.

Furthermore, at very low temperatures and high frequencies, a further relaxation process is observed, named local mode. This process is almost independent of the side chain length. Finally, the  process detected for all samples represents the conductivity of the samples. It is not a relaxation process of the polymer chains and thus it is not of interest for the present study. The conductivity of the sample increases with increasing side chain length.

Dynamic mechanical measurements.

The mechanical analysis of polymers was performed in a way typically used for amorphous polymers. The properties of all samples are characterized by the frequency dependencies of the real (G') and imaginary (G'') parts of the complex shear modulus determined in a dynamic deformation experiments with sinusoidal deformation signal. The reported dependencies are master curves obtained by only frequency shifts of the spectra recorded at various temperatures to a single reference temperature (20 °C common for all samples). Figure 3 Solid-state NMR. The NOE experiment with dipolar filter 3 used in the present work to quantify chain dynamics in poly(n-alkyl acrylates) consists of a dipolar filter, selecting the magnetization in the more mobile parts of the sample, followed by a mixing time during which magnetization is transferred in the sample by a cross-relaxation (or NOE) mechanism (see supporting information for pulse scheme). Due to the poor resolution of 1 H static spectra of polyacrylates at the temperatures investigated here, the observed 1 H spectra exhibit an apparent single line at almost all temperatures (above Tg + 90 K other signals start to be distinguishable). The spectrum recorded at the end of the dipolar filter exhibits an apparent single line, which is narrower than the apparent single line observed without dipolar filter.

Then for increasing mixing times m, the apparent single line becomes broader, until it reaches the line width observed without dipolar filter. Several aspects need to be checked prior to quantification of chain dynamics using this experiment: 3 is there a dynamic contrast inside the sample ? What selection does the dipolar filter actually do ? Is the mechanism for magnetization transfer during the mixing time actually cross-relaxation ?

The dynamic contrast (difference in mobility between the more mobile and the less mobile parts of a sample) was characterized in all samples by solid-state NMR 19 . 1 H static spectra exhibit a line which gets narrower in a visually homogeneous way with increasing temperature. 11 This means that the whole sample is becoming more mobile with increasing temperature, and exhibits no strong dynamic contrast.

In order to characterize more precisely the dynamic contrast, the 2D-WISE technique 15 was used. In a 2D-WISE spectrum, the different chemical groups of the molecule are resolved according to their 13 C chemical shifts in the 13 C dimension; furthermore, the line width in the 1 H dimension gives a rough indication of the mobility of the corresponding group: the narrower the line, the more mobile the chemical group. 2D-WISE measurements were done at Tg+70 K for all samples (see supporting information). In PMA, PEA and PBA the side chain end is significantly more mobile than the main chain, while in PHxA a mobility gradient is observed along the alkyl side chain, starting from the more mobile CH3 end group. 11 The selection done by the dipolar filter in the NOE experiment was checked on PEA and PBA by an independent NMR measurement: the selected 1 H signal was transferred to 13 C nuclei via Lee-Goldburg cross-polarization and acquired in the 13 C channel under MAS, to gain chemical shift resolution. 3 The obtained 13 C spectra are presented in supporting information. They prove that the dipolar filter actually selects the end of the alkyl side chains, namely only the CH3 end group and partly the next CH2 group(s). 11 The mechanism of magnetization transfer during the mixing time was investigated by monitoring the intensity of the more mobile parts of the sample as a function of the mixing time during the exchange experiment with dipolar filter, for different filters 3 . Two mechanisms are possible. A coherent magnetization transfer via residual dipolar couplings would result in a linear dependency of the intensity with the square of the mixing time. 20 A non coherent magnetization transfer via cross-relaxation (or NOE) results in a logarithmic dependency of the intensity on the mixing time. 3 Those plots were done for all samples at all temperatures, and the representative example of PEA at 329 K is shown in Figure 4. The logarithmic plot of recorded intensity versus mixing time is linear (in agreement with an NOE mechanism) while the plot of recorded intensity versus the square of the mixing time is clearly curved (indicating a negligible contribution of coherent magnetization transfer). Therefore it is concluded that the magnetization transfer during the mixing time occurs predominantly via a non coherent mechanism (although a very small contribution of coherent magnetization transfer can not be excluded). Thus the recorded data were treated and interpreted considering a cross-relaxation formalism for all samples at all temperatures. 3 Concerning the equation governing the magnetization transfer, the one-dimensional NOE experiment with dipolar filter conducted in the present work is equivalent to monitoring a diagonal line intensity in a 2D NOE experiment. 3 In the investigated samples, the dipolar filter selects the end of the alkyl side chains, namely only the CH3 end group and partly the next CH2 group(s). Therefore the equation corresponding to homonuclear CH3-CH2 system in the slow motion limit 3 was derived from general equations given by Macura and Ernst 21 :

( ) ( )   m C m 5 exp 2 3 K   q τ I - +  = (6)
where I(m) is the monitored line intensity, K a constant, q the coupling strength, C the correlation time molecular motion giving rise to the cross-relaxation, and the m the mixing time. The q parameter was independently measured through the second moment of the 1 H line recorded below Tg under static conditions 3 : 8.42 kHz 2 for PMA, 11.5 kHz 2 for PEA, 9.81 kHz 2 for PBA, 8.77 kHz 2 for PHxA (data not shown 11 ). It should be emphasized that a CH3-CH2 system does not strictly represent the whole monomeric unit, however, to our knowledge, no analytical equation is available for moieties larger than two groups of equivalent nuclei.

The correlation times C measured by NOE are plotted together with the ones of dielectric and mechanical relaxation for all samples (Figure 5). The relaxation process observed by the NMR experiment is detected on a timescale slower than the -relaxation (cooperative -relaxation) and at temperatures higher than the main chain simple -relaxation detected by dielectric spectroscopy as well as dynamic mechanical measurements at the same temperature (both exhibiting similar traces, Figure 5).

Individual larger scale Arrhenius plots for each sample and numerical values are given in supporting information.

Discussion

The relaxation process observed with the NMR experiment is detected on a timescale slower than the -relaxation (cooperative -relaxation) and at temperatures higher than the main chain simple relaxation detected by dielectric spectroscopy and dynamic mechanical measurements (both exhibiting similar traces, Figure 5). This relaxation process in poly(n-alkyl acrylates) is detected and quantified for the first time on such a time scale and temperature range. It should be underlined here that different relaxation processes may be detected by NOE with dipolar filter and by dielectric spectroscopy, due to different detection methods. The dielectrics indeed primarily detects motions via the permanent dipole located at the carbonyl group in the laboratory frame, while the NOE probes motions more locally via relative motions of neighboring 1 H nuclei causing a modulation of the dipolar coupling in their local molecular frame. The NOE experiment actually detects the motion of the group selected by the dipolar filter (predominantly the CH3 group) together with the group(s) to which it is transferring magnetization (the next CH2 group(s)), as a whole, with respect to their environment.

The poly(n-alkyl acrylates) investigated here exhibit a local nanostructure, as shown by WAXS results (see supporting information). The geometry of this local structure is not known precisely, except that it involves nanodomains of side chains. 8 Previously investigated poly(n-alkyl methacrylates) are similar in chemical nature and exhibit a layered structure, in which disordered sheets of aligned main chains are separated by layers of side chains. 22 By analogy with poly(n-alkyl methacrylates), the relaxation process observed by NMR in poly(n-alkyl acrylates) could be the isotropization of the main chain, which was recently detected for poly(n-alkyl methacrylates) on a time scale slower than the conventional -relaxation 17,23 . This isotropization process arises because the polymer melt is locally structured and because of the presence of extended backbone conformations, thus the isotropization of the backbone conformation needs many steps of restricted locally axial chain motion. 17,23 The NMR relaxation data together with the dielectric simple -relaxation data were fitted for all samples to a WLF equation 24 :

( )

g g Cg C T T C T T C - + - - = 2 1 log   (7)
where Cg is 100 s, Tg is the glass transition temperature, and C1 and C2 are the WLF coefficients.

Physically sound values were found for the fitted processes: C1 = 9 and C2 = 17 K for PMA, C1 = 9 and C2 = 15 K for PEA, C1 = 8 and C2 = 13 K for PBA, C1 = 8 and C2 = 20 K for PHxA (see supporting information for more details). Furthermore, the respective Tg values found by WLF fits (289, 257, 225 and 211 K) are in agreement with those measured by DSC (Table 1). Thus the relaxation process observed by NMR could be the isotropization of the main chain in poly(n-alkyl acrylates). However, the main chain isotropization relies on the main chain rigidity for polymethacrylates, yet polyacrylates have a significantly more flexible main chain. Thus the isotropization process for polyacrylates is expected to take place on a significantly faster time scale than for polymethacrylates. Moreover, for PBA and PHxA the correlation times measured by NMR at the lowest temperature (Tg+20 K) seem to fall outside of the common trace of dielectric spectroscopy and NMR fitted by the WLF equation, and at the crossover between the dielectric and NMR data, the apparent activation energies of the processes detected by each method are different. Furthermore, it might seem surprising that the relaxation process detected by NMR is slower but has a lower apparent activation energy than the -relaxation process detected by dielectric and mechanical spectroscopy, but this is also observed for the isotropization and -relaxation processes in poly(n-alkyl methacrylates). 17,23 Finally, the interpretation of the relaxation process detected by NMR in terms of main chain motion is questionable, due to the selection done by the dipolar filter at the beginning of the NMR experiment: only the end of the local side chain is selected. Therefore, the NMR technique is expected to detect a relaxation of the side chain, which seems to be in contradiction with the possible detection of the isotropization of the main chain.

Assuming that the NMR experiment would detect a local relaxation process within the side chain, it is consistent to assume an Arrhenius behavior for it. The resulting linear dependence of the correlation time C on inverse temperature is in agreement with the experimental data, although those data were acquired on a too narrow temperature range to allow any conclusion on a possible curvature. The experimental data were fitted to the Arrhenius equation for all samples:

        = RT E A a C exp  (8)
where A is the pre-exponential factor, Ea is the activation energy of the relaxation process, R is the gas constant, and T is the absolute temperature. The values found for pre-exponential factor and apparent activation energy were 210 -13 s and 22 kJmol -1 for PMA, 610 -13 s and 18 kJmol -1 for PEA, 610 -11 s and 13 kJmol -1 for PBA, 910 -11 s and 12 kJmol -1 for PHxA respectively. More details on linear regression of correlation and relaxation times for the process detected by NOE, as well as the relaxation process and the relaxation process labeled as "local" can be found in supporting information.

For PBA and PHxA, the activation energy of the process detected by NOE is intermediate between those of the local relaxation and of the -relaxation (10 kJ.mol -1 and 18 kJ.mol -1 respectively for PBA, 10 kJ.mol -1 and 15 to 25 kJ.mol -1 for PHxA). For PEA and PMA, the activation energies of the process detected by NOE are also higher than those of the local relaxation (ranging respectively from 8 to 14 kJ.mol -1 and from 11 to 19 kJ.mol -1 ). Please note that for PMA and PEA, no -relaxation process was detected by dielectric spectroscopy. However, it could be present but insufficiently resolved from the simple -relaxation process, due to proximity in frequency and a lower intensity of the -relaxation (it might be detectable by mechanical measurements with another geometry). The simple -relaxation is usually attributed to motions of the main chains, -relaxation (cooperative -relaxation) to motions of the main chains coupled to reorientations of the side chains, -relaxation to reorientations of the whole COO-alkyl side chains, and local relaxation to reorientations of parts of these side chains in poly(alkyl acrylates). [START_REF] Mccrum | Methacrylates and related polymers[END_REF] Thus the process detected by NOE could logically be assigned to reorientations of parts of the side chains, i.e. to a superposition of -relaxation and more local relaxation. The relaxation process detected by NMR would then be the continuation at high temperature of a local relaxation process usually observed at temperatures lower than that of the -relaxation. This would be in agreement with a lower activation energy than that of the -relaxation. Nevertheless, it might seem surprising that the corresponding motions of the side chain be slower than those of the main chain (-and -relaxations) in the temperature range where the measurements were carried out. However, in the context of locally organized samples investigated by different methods, it is conceivable that some side chain motions would be slower than those of the main chain. Observation of a relaxation process does not imply that such a process leads to complete relaxation of main chains and side chains. 17,23 This leaves room for detecting additional relaxation via another technique on longer time scales. Thus, time scale and amplitude of relaxation processes should clearly be distinguished (see, e.g., ref 26). Indeed, the -and -relaxation processes of the main chains are detected by dielectric (and mechanical) spectroscopy, while the new relaxation process is detected by NOE NMR. Dielectric spectroscopy primarily detects motion in the laboratory frame via the permanent dipole located at the carbonyl group. The NOE experiment probes motions more locally via relative motions of neighboring 1 H nuclei causing a modulation of the dipolar coupling in their local molecular frame. Therefore the -relaxation and the relaxation process probed by NOE are detected in different frames (respectively local molecular frame and laboratory frame). Furthermore, as detailed above, the poly(n-alkyl acrylates) investigated here exhibit a local nanostructure, which involves nanodomains of side chains. The existence of the local nanodomains restricts the motion of the involved side chains with respect to their neighbors. Thus the side chains might reorient within the nanodomain in the local molecular frame on a time scale longer than that of the main chain -and -relaxation in the laboratory frame. The NOE experiment would then detect hindered motions of the side chains inside the nanodomains in the local molecular frame, resulting from -relaxation and more local relaxations.

Finally, an interesting property was observed for the relaxation process measured by NMR. Although the measured correlation times depend on the alkyl side chain length, they become sample-independent when plotted as a function of the temperature difference to Tg or of the inverse temperature by Tg (Figure 6).

Conclusions

Through NMR measurements with a recently developed solid-state NMR method (NOE with dipolar filter), a relaxation process was detected in various members of the poly(n-alkyl acrylates) family, in the melt. This relaxation process occurs at a higher temperature than does the simple -relaxation and on a longer time scale than that of the -relaxation (cooperative -relaxation). It was detected and quantified for the first time on these temperature and time scales.

Due to the presence of branching in polyacrylates, dielectric spectroscopy and dynamic mechanical measurements were conducted on the same samples to help interpreting the NMR results. The relaxation process detected in polyacrylates could be the isotropization of the main chain, by analogy with structurally similar poly(n-alkyl methacrylates). It is more probably a local relaxation within the alkyl side chain, that is usually detected only at significantly lower temperatures. The fact that this side chain motion is slower than the cooperative main chain -relaxation is striking, but can be rationalized in the context of locally nanostructured samples investigated by methods detecting motion in different frames.

A definitive molecular assignment of the relaxation process detected by the NOE NMR method requires further investigation. It could be done by advanced solid-state NMR techniques with selectively 2 H or 13 C labeled samples. The experimentally measured values are plotted together with literature values from dielectric relaxation spectroscopy: Beiner et al. 8 (◯), Buerger et al. 27 (-, PMA), de Brouckere et al. 28 (--), Fioretto et al. 29 (⊗ ), Fitzgerald et al. 30 (-, PBA), Gomez-Ribelles et al. 31,32 (•), Hayakawa et al. 33 (■,--, PBA), Jourdan et al. 34 (□), Kahle [START_REF] Kahle | alpha and beta-relaxation in poly(methyl acrylate)[END_REF] (■, PMA), Mead et al. [START_REF] Mead | [END_REF] (⊝ ), McCrum [START_REF] Mccrum | Methacrylates and related polymers[END_REF] (⊖ ), Reissig [START_REF] Reissig | Dielektrische und kalorimetrische Untersuchungen des Einflusses molekularer Details auf das -Splittingsverhalten der Poly(n alkylmethacrylat)e[END_REF] (⊕ ); and from mechanical spectroscopy: Gomez-Ribelles et al. 32 (▲), Soen et al. [START_REF] Soen | [END_REF]

(△). Larger plots for individual

samples are available in supporting information.

WAXS investigation of structure in the studied samples

Experimental. WAXS measurements were performed using a - diffractometer (Siemens). Cu K radiation (=0.154 nm) was used. Measurements were performed at room temperature. The recorded scattered intensity distributions are presented as functions of the scattering vector (s=2sin/, where 2

is the scattering angle).

Results. Constitution and arrangement of atoms in monomeric units as well as arrangement of the neighboring chains in bulk amorphous polymers result in variation in intensity distribution detected by the scattering of X-rays in the wide angle range (WAXS). The figure below compares WAXS intensity distributions for the studied polymers. In all cases, the intensity distributions consist of two broad to some extent overlapping peaks with positions of maxima dependent on the monomer type. The correlation distance corresponding to the low angle peak linearly increases with the alkyl side group length, as shown in the insert. The second maxima in the scattered intensity distributions do not differ much between the samples studied.

Discussion. Diffraction patterns for poly(n-alkyl acrylates) were measured previously by Heijboer et al. 1 and Beiner and Huth 2 . Heijboer reported the peak corresponding to Van der Waals (VDW) distances between non bonded atoms, nearly independent of the alkyl side chain length, and both reported the peak corresponding to distances larger than Van der Waals (LVDW) distances, increasing with alkyl side chain length. The observation of the LVDW peak is an indication of a local structure in poly(nalkyl acrylates) in the melt. The geometry of this local structure is not known precisely, except that it involves nanodomains of side chains. 2 However, a local structure has been characterized in the chemically similar poly(n-alkyl methacrylates). 3 In poly(n-alkyl methacrylates), the temperature dependent observation of three peaks allowed to elucidate a layered structure, in which disordered sheets of aligned main chains are separated by layers of side chains. 3 It should be emphasized that this local structure occurs on the length scale of 5 to 10 monomeric units. 4 The observation in poly(n-alkyl acrylates) of two peaks in common with poly(n-alkyl methacrylates) suggests a similar but less organized local structure. The LVDW peak is attributed to a backbone-backbone correlation in the amorphous bulk material and the VDW peak to interatomic distance correlations which depend on the monomer constitution and to a large extent on the intramolecular arrangement of atoms.

It is tempting to deduce information on the conformation of the side chain in the local structure from the dependency of the LVDW distance upon alkyl side chain length. Indeed, an extended conformation of the side chains would result in an affine dependency of the LVDW distance upon alkyl side chain length, while a Gaussian coil conformation of the side chain would result in an affine dependency of the square root of the LVDW distance upon alkyl side chain length. 5 However, even if an affine dependency of the LVDW distance upon alkyl side chain length is observed, the chains can fill space with not completely extended conformations, as shown in the case of substituted polyamide rigid rods. 6 (--), Gomez-Ribelles et al. 13,14 (•), Kahle 15 (■), Mead et al. 16 (⊝ ), McCrum et al. 17 (⊖ ), Reissig 18 (⊕ ); and from mechanical spectroscopy: Gomez-Ribelles et al. 13 (▲), Soen et al. 19 (△).

  m           +x -y +x +x -y +x -x +y -x -x +y -x +x x (a) (b) (c)

Figure 3

 3 Figure 3 bottom by means of the vertical solid lines.

Figures Figure 1 Figure 1 .

 11 Figures

Figure 2 Figure 2 .

 22 Figure 2

Figure 3 Figure 3 .

 33 Figure 3

  ); from left to right: PMA, PEA, PBA, PHxA.

Figure 4 Figure 4 .

 44 Figure 4

Figure 5 Figure 5 .

 55 Figure 5

Figure 6 Figure 6 .

 66 Figure 6

Figure .

 . Figure. X-ray diffraction intensity distributions for the studied amorphous polymers: PMA (solid black

Figure . 3 MCFigure. 13 C

 .313 Figure.Pulse scheme of the NOE experiment with dipolar filter, consisting of a dipolar filter combined with a conventional exchange experiment.10 At the end of the dipolar filter (a), only the more mobile1 H nuclei of the sample possess magnetization. Then the magnetization remaining in the xy-

  Figure.Arrhenius plot of the correlation times extracted for sample PMA using solid-state NMR (+),

Figure .

 . Figure.Arrhenius plot of the correlation times extracted for sample PEA using solid-state NMR (+),

Figure .

 . Figure.Arrhenius plot of the correlation times extracted for sample PBA using solid-state NMR (+),

Figure . Figure .

 .. Figure. Arrhenius plot of the correlation times extracted for sample PHxA using solid-state NMR (+),

Table 1

 1

		PMA	PEA	PBA	PHxA
	Tg (K)	294	259	227	213
	Mn (g•mol -1 )	43,000	163,000	100,000	88,500
	Mw (g•mol -1 )	229,900	341,000	318,000	395,000
	Mw/ Mn	5.3	2.1	3,2	4.5

Table 1 .

 1 Characterization of the different poly(n-alkyl acrylate) samples.

scheme of the NOE experiment with dipolar filter n

	Pulse
	PMA				35 ppm
					40 ppm
					50 ppm
	-4	-2	0	2	4	kHz
	PEA				35 to 40 ppm 14 ppm
					60 ppm
	-4	-2	0	2	4	kHz
					14 ppm
	PBA				19 ppm 30 ppm
					40 ppm
					65 ppm
	-4	-2	0	2	4	kHz
	PHxA				14 ppm 25 to 30 ppm 40 ppm
					65 ppm
	-4	-2	0	2	4	kHz
	Figure. 1D 1 H spectra extracted from the 2D-WISE spectra 7 of model PnAAs at Tg+70 K ( 1 H Larmor
	frequency of 300.13 MHz, static, Lee-Goldburg cross-polarization 8 and -pulse during t1). The chemical
	shift assignment is given below.
	 (ppm) Assignment (PMA) 9	 (ppm)	Assignment (PEA) 9
	35	CH2 (SC)		14		CH3 (SC)
	42	CH (MC)		35		CH2 (MC)
	52	O-CH3 (SC)		42		CH (MC)	Assignment
	175  (ppm) Assignment (PBA) 9 C=O 14 CH3 (SC)	60 175  (ppm)	O-CH2 (SC) C=O Assignment (PHxA) 9	of the 13 C chemical shifts of PnAAs (MC:
	19 30 35	CH2-CH3 (SC) CH2-CH2-CH3 (SC) CH2 (MC)	14 22, 26, 29, 32 CH2 (SC, except OCH2) CH3 (SC) 35 CH2 (MC)	main chain, SC: side chain).
	40	CH (MC)		42		CH (MC)
	64	O-CH2 (SC)		65		O-CH2 (SC)
	175	C=O			175	C=O

  12

Table .

 . Results of WLF fits of the some relaxations processes of PnAAs detected by NMR and dielectric spectroscopy.

	Sample	Process	C1	C2 (K)	Tg (K)
	PMA	NMR + dielectrics	9.3  0.3	17  2	289  1
		dielectrics	20  1	76  6	285  1
	PEA	NMR + dielectrics	9  4	15  2	257  1
		dielectrics	15  1	42  3	255  1
	PBA	NMR + dielectrics	8.1  0.2	13  2	225  1
		dielectrics	9.1  0.4	19  3	224  1
	PHxA	NMR + dielectrics	8.8  0.2	20  4	211  1
		dielectrics	9.2  1	26  5	210  1

Prefactor and activation energy for the -relaxation, local

relaxation and relaxation detected by NOE

	Sample	Process	logA	A (s)	Ea/R Ea (kJ.mol -1 )	R 2	ref.
	PMA	local relaxation	-13.0 9  10 -14 1.46	12.1	0.997	p.w.
		(dielectrics)	-13.7 2  10 -14 1.69	14.1	0.999	18
			-13.9 1  10 -14 2.26	18.8	0.999	15
			-11.2 6  10 -12 1.28	10.7	0.994	15
			-13.4 4  10 -14 1.78	14.8	1	11
			-12.9 1  10 -13 1.75	14.6	0.991	17
			-10.3 5  10 -11 1.39	11.6	1	14
		NOE	-12.7 2  10 -13 2.65	22.0	0.998	p.w.
	PEA	local relaxation	-11.6 2  10 -12 0.97	8.1	0.982	p.w.
		(dielectrics)	-14.4 4  10 -15 1.52	12.6	0.998	18
			-9.1 9  10 -10 0.98	8.1	0.995	14
			-12.8 2  10 -13 2.00	13.5	0.999	17
		NOE	-12.2 6  10 -13 2.17	18.0	0.994	p.w.
	PBA	-relaxation	-13.8 2  10 -14 2.17	18.0	0.982	p.w.
		(dielectrics)	-13.0 1  10 -13 2.11	17.5	0.993	2
		local relaxation	-13.3 5  10 -14 1.21	10.1	0.987	p.w.
		(dielectrics)	-12.8 2  10 -13 1.20	10.0	0.996	18
		NOE	-10.2 6  10 -11 1.61	13.4	0.997	p.w.
	PHxA	-relaxation		no satisfying fit possible,		2 , p.w.
		(dielectrics)	Ea in the range from 15 to 25 kJ.mol -1
		local relaxation	-13.1 7  10 -14 1.22	10.2	0.997	p.w.
		(dielectrics)
		NOE	-10.1 9  10 -11 1.49	12.3	0.993	p.w.

Table .

 . Prefactor A and activation energy Ea for the relaxations processes detected in model PnAAs which follow (, local) or could follow (NOE) an Arrhenius behavior; the abbreviations R 2 and p.w. designate respectively the coefficient of determination and the present work.

Acknowledgements. M.G. thanks Arkema and Total for financial support. Dr. Patrice Castignolles is thanked for multidimensional SEC measurements, Andreas Hannewald for measurements of mechanical spectroscopy.

"Supporting Information Available: experimental conditions, figures and tables. This material is available free of charge via the Internet at http://pubs.acs.org."

For Table of Contents use only

Detailed experimental conditions for the sample characterization

The glass transition temperature (Tg) of the polymers were obtained using differential scanning calorimetry (DSC) on a Mettler Toledo Star System. The measurements were done at a nitrogen flow of 30 mL•min -1 , with the following temperature cycle: heating from -100 °C to 150 °C at 10 °C•min -1 , then cooling from 150 °C to -100 °C at 10 °C•min -1 , and finally heating from -100 °C to 150 °C at 10 °C•min -1 . The first heating and cooling steps are used to erase the thermal history of the sample and detect evaporation of little molecules possibly trapped in the samples. The measurements were done on the second heating step.

The SEC was done in Paris with a Triple Detector Array (TDA, from Viscotek) equipment composed of an online degasser, pump, manual injector, a precolumn and three columns (two mixed-C and one 10 2 Å) from Polymer Lab., the TDA (including in serie RALLS, LALLS at 7°, refractometer and finally viscosimeter), the software Trisec 2000. Eluent was THF at 40 °C and 1 mL.min -1 .

2 13 C solution-state NMR spectra of the model PnAAs dissolved in CDCl3 were recorded on a Bruker DRX500 at a 13 C frequency of 125.76 MHz. The spectra were recorded at 25 to 33 °C. Single pulse excitation with a 6.70 µs 90° pulse and inverse gated decoupling was used with a relaxation delay of 10 s to record quantitative spectra 19 500 to 21 000 transients were recorded. The ppm-scale was calibrated with the middle line of CDCl3 at 77.00 ppm.

Thermogravimetric analysis (TGA) was carried out on a TG 50 Mettler device under nitrogen atmosphere for each sample; the temperature was increased from room temperature to 900 °C at 10 °C•min -1 . The sample mass was recorded during this increase, and the left limit of its decrease was measured: it is the temperature at which molecules begin to evaporate from the samples (detected as the loss of one percent of the mass), indicating sample decomposition. The measured values, respectively Tg+206 K, Tg+248 K, Tg+288 K, Tg+307 K for PMA, PEA, PBA and PHxA, can not be used to predict the stability of the samples at high temperatures. Indeed, decomposition of the PBA and PHxA samples was observed during NMR measurements after several hours at ca Tg+150 K.

Correlation times measured with NOE experiment with dipolar filter

Sample q (kHz 2 ) T (K) T-Tg (K) qC (Hz)

C


Patrice Castignolles
email: pcastignolles@usyd.edu.au

Marianne Gaborieau

Emily F Hilder
email: emily.hilder@utas.edu.au

Ewan Sprong

Christopher J Ferguson

Robert G Gilbert
email: gilbert@chem.usyd.edu.au

High-resolution separation of oligo(acrylic acid) by capillary zone electrophoresis

Keywords:

come

Introduction

Homopolymers and copolymers of acrylic acid (AA) have numerous diverse applications, from the large-scale production of surface coatings and superabsorbants [START_REF] Buchholtz | Polyacrylamides and Poly(Acrylic Acids)[END_REF] to the high-tech additives for controlled mineralization, nanoreactors, drug carrier systems. [START_REF] Colfen | [END_REF] However the free-radical polymerization of AA is very complex [3,4] and the characterization of AA-based polymers is difficult. [5] Nevertheless, some researchers have been able to control molar masses in the free-radical polymerization of AA using RAFT agents [6] and nitroxides, [7] which have led to well-defined (although not uniform i.e. not "truly monodisperse") polyor oligo(acrylic acid) chains. The controlled synthesis of oligo(acrylic acid), oligoAA, by RAFT is a key step in the synthesis of new types of coatings. [8] These oligomers are also good models to test and compare different possible characterization methods for poly(acrylic acid) (PAA).

Aqueous size exclusion chromatography (SEC) is currently the most widely used technique for the separation of synthetic polyelectrolytes, but has many difficulties. The separation assumed to be based on size exclusion proves to be strongly disturbed by adsorption phenomena, [5] and there is some controversy about the SEC of PAA. Aqueous SEC of PAA and SEC of methylated PAA in tetrahydrofuran yield significantly different molar mass distributions, [9] while dimethylacetamide has also recently been proposed as an eluent. [5] While capillary electrophoresis (CE) is widely used to separate biomacromolecules, its application to synthetic polymers is recent. [10] In the case of synthetic polyelectrolytes, CE proved to give higher resolution than electrophoresis. [11][12][13][14] Oligo(styrene sulfonate), oligoSS, can be completely resolved by capillary zone electrophoresis (CZE) up to a degree of polymerization Nmax of 9. [15] The separation of high molar masses has been conducted using capillary gel electrophoresis, CGE, [10,[START_REF] Engelhardt | Characterization of synthetic polyelectrolytes by capillary electrophoretic methods[END_REF] leading to a better resolution in a shorter time than SEC.

No separation of PAA had been attempted using CZE. [START_REF] Hoagland | [END_REF] CGE coupled to mass spectrometry has been applied to oligoAA, [18] but the separation was poor, with identification based on mass spectrometry.

The objective of the present Communication is to show that CZE could be applied successfully to the quantitative analysis of the molar mass distribution of this oligomer.

Experimental

Oligomerization of Acrylic Acid

Four oligoAAs were synthesized targeting different theoretical molar masses and the synthesis by RAFT polymerization described in detail elsewhere. [8]

Electrospray Ionization Mass Spectrometry (ESI MS)

The experimental conditions for ESI MS analysis of oligoAA have been described elsewhere [8] and are repeated in the supporting information. The four oligoAAs are denoted by the degree of polymerization N corresponding to the maximum intensity on the ESI spectra: AA2, AA5, AA7 and AA15. Figure 1 shows the ESI MS spectrum of AA15; that of an oligoAA very similar to AA5 was shown previously, [8] while that of AA2 is given in the supporting information.

According to the ESI MS spectra, the majority of the oligoAA chains have the chemical structure depicted in Figure 1 (see discussion in ref [8] ). Most chains should have the RAFT agent-derived propanoic acid group at one end and the remaining part of the RAFT agent at the other. It is emphasized that ESI MS (without chromatographic separation) suffers from mass discrimination: [19] it allows very accurate measurement of molar masses but does not give the true molar mass distribution, and some higher molar masses might be present but undetected.

Size Exclusion Chromatography

The SEC and conditions described previously by Ferguson et al. [8] for acrylic acid based block oligomers were applied to the present oligoAA.

Capillary Electrophoresis

CZE experiments were carried out using an Agilent 3D CE system (Agilent Technologies, Waldbronn, Germany) equipped with a diode array detector. The capillary was thermostatted at 25 °C for all separations. Polyimide-coated fused silica capillaries (50 m i.d., Polymicro Technologies, Phoenix, AZ, USA) of total length 40 cm (31.5 cm to the detector) were pretreated by rinsing for 5 min with 1 mol•L -1 NaOH before use. Buffers were prepared from 20-1000 mmol•L -1 boric acid, titrated to pH 9.2 with one of lithium, sodium or potassium hydroxide. AA5, AA7 and AA15 were first dissolved in pure water at a concentration of 10 g•L -1 , then diluted with buffer and water, in order to reach concentrations between 0.5 and 7 g•L -1 (the concentration leading to optimal resolution increases with N). Due to the reduced solubility in water, the RAFT agent and AA2 were dissolved in 50 m mol•L -1 lithium borate at 10 g•L -1 then diluted to 0.5 g•L -1 with water. All injections were hydrodynamic, i.e. pressure (50 mbar) was applied during the injection time (between 10 and 20 s) on the vial containing the sample and the capillary end. Detection was at 200, 250, 290 and 340 nm (see supporting information) with 290 nm corresponding to the maximum absorption in the UV spectrum of the RAFT agent. [20]

Results and discussion

Separation by Capillary Zone Electrophoresis, Comparison with SEC

With a normal cathodic electroosmotic flow, EOF, separation of the oligoAA samples was achieved in a counter-EOF mode as shown in Figures 234. The electrophoretic mobility of each of the oligoAAs is, as expected, negative and is also weaker than the EOF (see supporting information). This is consistent with the same observations made for oligoSSs (using CZE with bare fused silica capillaries). [15] The separation of these oligoAAs was also carried out by SEC (see insert in Figure 3 and supporting information) with an eluent composed of THF and acetic acid (and alternatively an aqueous disodium phosphate buffer).

The SEC resolution was significantly lower than that of CZE. In fact, to our knowledge, SEC of oligoelectrolytes has never achieved oligomeric resolution.

Water-soluble oligomers based on AA had been thoroughly investigated with a SEC system different from that described here: [21] aqueous SEC with 0.01 mol•L -1 NaBr; this yielded only monomodal chromatograms. The high resolution separation achieved by CE indicates that this is a highly suitable technique for understanding the oligomerization taking place in the aqueous phase during emulsion polymerization involving AA, which is common industrially, and indeed any water-soluble oligomers. [22] The oligomerization process does not necessarily involve only charged monomers, and CE can also be applied to non-charged monomers, especially if the initiator, typically potassium persulfate, leads to end-charged oligomers. Isotachophoresis has previously been used to characterize water-soluble oligomers produced during emulsion polymerization of styrene (i.e. no water soluble comonomer being present). [23] Since then, significant progress have been achieved in CE, including a detailed study of the separation of end-charged synthetic polymers by the group of Cottet. [24]

Repeatability of the Separation

The electropherograms obtained were qualitatively repeatable (as long as the current is not too high), although migration times were not systematically highly repeatable even after electro-osmotic flow correction (see supporting information for repeatability tests and electroosmotic flow correction). This problem can be overcome by using two electrophoretic markers, i.e. not only the peak corresponding to electro-osmotic flow but also a marker whose electrophoretic mobility is higher than any of the oligoAAs and which would thus migrate in the last position. Possible explanations are slight adsorption of oligoAA on the capillary walls and/or a strong Joule heating effect. In the case of PAA, Hoagland [START_REF] Hoagland | [END_REF] considered that there is no adsorption on the capillary wall in a similar system to the present, but observed a strong Joule heating effect. The studies of Garcia et al. [18] using CGE did not use thermostatted capillaries, suggesting that the authors did not consider Joule heating to be a significant effect, but no information was given on the repeatability of the separations. In the present paper work it is assumed that some slight adsorption of oligoAA on the capillary walls takes place and the repeatability should thus be greatly improved by using suitable coated capillaries to eliminate this interaction. It is also expected that Joule heating will play a role, since the currents are very high (typically 150 A, see Figures 2 to 4) due to the very high buffer concentration that has been used. [START_REF] Foret | Capillary Zone Electrophoresis[END_REF] Despite this Joule heating, the increase in resolution due to the decreased EOF compensates for any reduction in the peak efficiency, and thus there is no deleterious effect on the overall separation.

Influence of the Buffer Concentration and Counterion

Figure 2 shows that an increase in buffer concentration leads to lower electrophoretic mobilities and thus longer migration times, a phenomenon also observed by Cottet et al. [15] for oligoSS separation. The best separation of oligoSS was obtained in pure water. The EOF decreases when buffer concentration increases; thus, in this counter-EOF mode, resolution should increase together with the separation time. [START_REF] Foret | Capillary Zone Electrophoresis[END_REF] This is what we observed with oligoAA: increasing the buffer concentration increases the resolution. An interesting alternative would be to use coated capillaries for oligoAA separation, in order to entirely eliminate the EOF, and then perform the separations in a negative electric field (i.e. anodic EOF) and with significantly lower buffer concentrations (improving then the repeatability).

Borate buffers were prepared using three different counterions: lithium, sodium or potassium. Using either potassium or sodium borate, similar separations were observed, with the difference in the separations probably due to differences in conductivities. Using a too high buffer concentration leads to excessive current and no separation; therefore, at the same buffer concentration, sodium borate gives higher resolution than potassium borate, because it induces a lower current for the same buffer concentration. On the contrary, lithium borate gives a qualitatively different separation from the previous two (see Figure 3 and supporting information). All separations obtained with lithium borate exhibit two sharp well resolved peaks followed by several partially resolved peaks; the separation with sodium or potassium borate shows three more resolved in between. Such changes in the separation selectivity have also been observed in other synthetic polyelectrolytes, namely N-phenylaniline oligomers, using non-aqueous CE. [START_REF] Cottet | [END_REF] In the case of oligoAA, this might be linked to differences in chain conformation (see next paragraph) or ionization [START_REF] Hoagland | [END_REF] , depending on the counterion. To discuss the mechanism of the separation, we focus on the separation using lithium borate, since it is the most straightforward to make a preliminary assignment.

Mechanism of the Separation with Lithium Borate

Figure 4 shows electropherograms obtained for the different oligoAA samples and the RAFT agent. The RAFT agent was identified by injecting it alone (bottom curve on figure 4); it migrates first. Using a high concentration of RAFT agent during the polymerization, an oligoAA composed mainly of N =1 and 2 was obtained (see ESI MS in supporting information). On figure 4, comparing AA2 with the others, the second peak is assigned to N = 1, followed by migration of the highest molar masses, represented by AA15. The peaks for oligomers with intermediate N can not accurately be assigned at this time, however Figure 4 shows that the oligomers with N > 1 migrate after the highest N.

In the case of oligoSS, oligomers with N values from 1 to 9 were detected with increasing migration times. [15] The separation mechanism was determined by measuring the evolution of electrophoretic mobility (ep) with molar mass. ep increases rapidly for degrees of polymerization below Nmax, then decreases slowly and reaches a constant value around Nplateau ≈ 100 for PSS. [27] The initial increase of ep may be linked to a rod-like conformation for the short oligomers in solution. The increase of the friction with increasing N is slower for a rod-like conformation than for a random coil conformation. Thus, as long as the rigid-rod conformation holds, ep increases rapidly with charge, i.e. with the degree of polymerization in the case of a homopolyelectrolyte. Increasing N leads to a progressive change from a rodlike to a coil conformation and thus to progressive higher friction: ep increases more slowly then decreases. ep reaches a constant value at high N values when the oligomer is in the coil conformation for which the increase of friction and charges counterbalance. The mechanism appears to be qualitatively the same for oligoAA. However, the oligomers of N = 15 were detected after the oligomers of N > 6 in the case of the oligoSS. With oligoAA, the oligomers of N = 15 are detected before the oligomers of N = 2. Nmax may thus be lower for oligoAA than for oligoSS. This difference may be explained by the difference of rigidity between the polyacrylate and PSS backbones. The polystyrene backbone is far less flexible than that of the polyacrylate, as indicated by the difference in glass transition temperatures of uncharged homologues. [START_REF] Kine | Acrylic and methacrylic acid ester polymers[END_REF][START_REF] Andrews | Glass transition temperatures of polymers[END_REF] Thus the very short oligoAA should behave like a rigid rod below a Nmax significantly lower than that of oligoSS.

Conclusion

OligoAAs have been separated for the first time by capillary zone electrophoresis. The mechanism of the separation is qualitatively the same as for oligoSS. The separation is complex (the detection order goes back and forth against N), but it has a high resolution that has never yet been achieved by size exclusion chromatography (for oligomers). These results open the way to accurate and high resolution characterization of oligoAAs and of block copolymers such as oligo(AA-b-styrene) which play a key role in RAFT emulsion polymerizations [8] . While CZE can be used to characterize oligoAA, the use of capillary electrophoresis can be extended to the separation of higher molar masses using capillary gel electrophoresis. Electro-osmotic flow is marked by a vertical arrow. In the case of AA2, an effective mobility correction was carried out using the N = 1 peak.

Figures

Table of Content (5 lines)

Oligomers of acrylic acid have been obtained by RAFT polymerization and separated for the first time by capillary zone electrophoresis (CZE). Borate buffers give a higher resolution than can be obtained by Size Exclusion Chromatography (SEC). The mechanism of the separation is discussed on the basis of the work of Cottet and Gareil on oligo(styrene sulfonate)s. Electrophoretic mobilities have been calculated using the relation:

-O -O - O - O - O - K + K + K + K + K + K + Electroosmotic velocity O O - + Electrophoretic velocity = Apparent velocity E O -O -O - O - O - O - K + K + K + K + K + K + O -O -O - O - O - O - K + K + K + K + K + K + O -O -O - K + K + K + O -O -O - O - O - O - K + K + K + K + K + K + O -O - K + K + O -O -O - O - O - O - K + K + K + K + K + K + Electroosmotic velocity O O -- + Electrophoretic velocity = Apparent velocity E O -O -O - O - O - O - K + K + K + K + K + K + O -O -O - O - O - O - K + K + K + K + K + K + O -O -O - K + K + K + O -O -O - O - O - O - K + K + K + K + K + K + O -O - K + K +
( ) eo M ep t t V L l m 1 1 -   =
where l is the length between the inlet and the detection windows of the capillary (here 31.5 cm), L is the total length of the capillary (here 40 cm), V is the electric field, t is the migration time of the sample and teo is the migration time of the neutral species.

Figure 1 :

 1 Figure 1: ESI MS spectrum of AA15. The numbers on top of the peaks are the degrees of

Figure 2 :

 2 Figure 2: Electropherograms of AA5 at 0.5 g•L -1 in sodium borate for three different buffer

Figure 3 :

 3 Figure 3: Electropherograms of AA5 with the two types of buffer: (a) sodium borate (400

Figure 4 :

 4 Figure 4: Electropherograms in lithium borate 500 mM (20 kV) for the four different

Figure : Figure 1 :Figure 2 :

 :12 Figure: Electropherograms of different acrylic acid (AA) oligomers obtained by capillary zone electrophoresis

Figure 3 :

 3 Figure 3: refractometer and right-angle light scattering traces of AA5, AA7 and AA15.Aqueous SEC with 0.1 mol•L -1 Na2HPO4. Shimadzu-Wyatt system (fully described in S.A.

Figure 4 :

 4 Figure 4: Electropherogram of AA5 (0.5 g . L -1 ) with the two types of buffer: potassium borate

Figure 5 :

 5 Figure 5: Mechanism of our separation by free solution Capillary Electrophoresis

Figure 6 :

 6 Figure 6: Repeatability of the injections. On the left, the electropherogram of two repeatability tests. On the right, the same plot but with electrophoretic mobilities.

Figure 7 :

 7 Figure 7: Comparison of the electropherograms obtained at different wavelength: 200 nm (blue, top), 250 nm (green, middle), 290 nm (red, bottom). The electropherograms obtained with AA5 at 0.5 g•L -1 , in lithium borate at 500 mmol•L -1 , V = 20 kV, I = 187 A. The different wavelengths give qualitatively similar electropherograms. However, 250 nm yields the least intense signal. 200 nm yields equivalent intensity; however, at this wavelength not only the RAFT agent but also the acrylic acid unit may absorbs, leading to very complex situation.

Figure 8 :

 8 Figure 8: ESI MS of AA2. For all the samples in this article, the conditions are the same as the ones given in Ferguson, C. J.; Hughes, R. J.; Nguyen, D.; Pham, B. T. T.; Gilbert, R. G.; Serelis, A. K.; Such, C. H.; Hawkett, B. S. Macromolecules 2005, 38, 2191: electrospray Mass Spectrometer Analysis was carried out using a Finnigan Mat LCQ MS detector with Finnigan LCQ Data Processing and Instrument Control Software. Ten microgram samples were dissolved in 10 mL of 50:50 methanol/H2O and fed into the electrospray ionization unit at 0.2 mLmin -1 . The electrospray voltage was 5 kV, the sheathing gas was nitrogen at 415 kPa, and the heated capillary was 200 °C.

Acknowledgements

Assistance from and discussions with Dr Michael Breadmore for the CE experiments are gratefully acknowledged, as are helpful discussions with Dr. Brian Hawkett. The Key Centre for Polymer Colloids was established and supported under the Australian Research Council's Research Centres Program. PC gratefully acknowledges an ARC International Linkage Fellowship. The second referee is thanked for useful comments.

Supporting information for

High-Resolution Separation of Oligo(acrylic acid) by

Capillary Zone Electrophoresis

Patrice Castignolles*, Marianne Gaborieau, Emily F. Hilder, Ewan Sprong, Christopher J.

Ferguson, Robert G. Gilbert