Thermal Cloud Point Fractionation of Poly(vinyl alcohol- co -vinyl acetate): Partition of Nanogels in the Fractions

: Poly(vinyl acetate- co -vinyl alcohol) (PVA), well-known as emulsion stabilizers, are obtained by partial hydrolysis of poly(vinyl acetate) (PVAc). Their thermal cloud point fractionation was performed in aqueous medium between 40 and 75 °C. This fractionation was carried out in order to get an insight in the partition of the initially present nanogels in the different fractions. All the fractions were characterized by size exclusion chromatography (SEC), NMR and dynamic light scattering (DLS) giving access to average degree of polymerization DPw , average degree of hydrolysis DH , average sequence lengths of vinyl acetate VAc0 n , volume fraction and average size diameter (Dv) of nanogels and “free PVA chains”. The polydispersity of the samples in DPw , DH and VAc0 n could be confirmed. The nanogels characterized by the highest values of volume fraction and Dv, in the range of 40–43 nm, were separated in the first coacervate fraction, whereas the most soluble fraction with low VAc content does not contain nanogels but only “free chains” of a Dv value of around 7–8 nm. The nanogels in the various fractions could further be disaggregated into “free chains” by complex formation with sodium dodecyl sulfate (SDS).


Introduction
Poly(vinyl alcohol-co-vinyl acetate) copolymers, currently referred as PVA, that are obtained by partial hydrolysis of poly(vinyl acetate) PVAc, are well known as stabilizers for the emulsion and suspension polymerization processes of various vinyl monomers such as styrene, vinyl acetate or vinyl chloride [1,2].The molecular characteristics of PVA's, mainly their polymerization degrees DPn , DPw and the average degree of hydrolysis DH , in general in the range of 70 to 90 mol%, have a major influence on the monomer droplet size, the dispersion stability and on the properties of the final PVC resins [3,4].
An additional feature is that small colloidal particles, so-called nano-or microgels or "pseudo-micelles", which are generally present in PVA's may have an influence on their emulsifying and stabilizing efficiency.As shown by different authors [5][6][7][8] these PVA nano-or microgels are formed by intermolecular hydrophobe-hydrophobe interactions for PVA's having DH values around 70-90 mol%.Furthermore, it should be mentioned that PVA's, used in industrial practice, are polydisperse in molar mass and in composition.For instance, Dawkins et al. [9] have clearly demonstrated by reversed phase liquid chromatography, that commercial PVA samples with an average DH value of 72 mol% contain in fact PVA chains with DH values ranging from 65 to 85 mol%.Zilberman et al. [10] came to a similar conclusion by cloud point fractionation, e.g., by temperature induced phase separation, of an aqueous PVA solution ( DH = 72.5 mol%).According to these authors it turned out that the average sequence distribution of vinyl acetate (VAc) and vinyl alcohol (VOH) monomer units, usually designated by "blockiness", has a major influence on the thermal phase separation and its compatibility with other water-soluble polymers.A typical example is that of hydroxypropyl methylcellulose, which is used in combination with PVA's in emulsifier formulations.
Another aspect of the PVA nanogels is their possibility to be disaggregated by complex formation with anionic surfactants; such as sodium dodecyl sulfate (SDS), as shown by Lewis and Robinson [6], Aladjoff et al. [11] and by Meehan et al. [12].
The aim of the present study was to take advantage of the cloud point fractionation technique, a process which does not need any additional solvent or precipitant, in order to check the "blockiness" index and the partition of the nanogels, especially their size and their relative volume fraction, in a fractioned PVA.In order to confirm the presence of nanogels in the different PVA fractions, this study will be completed by checking the influence of PVA/SDS complex formation on the disaggregation of these nanogels.These characteristics could be of interest for the optimization of the emulsifying efficiency of PVA's, by adjusting the saponification conditions of PVAc precursors.

Materials
The PVAs examined in this study were supplied by Nippon Gohsei and Synthomer under the trade names KP08 and B72 respectively.These samples, hereafter used without further purification, are identified by their average DH and DPw , as for instance PVA-73-650 and PVA-73-685 for sample B72 and KP08 respectively.The main characteristics of the PVA's, determined by 1 H NMR and SEC, are summarized in Table 1.The average hydrolysis degree, DH with a precision of ±0.5 mol%, was determined using 1 H NMR (Bruker AC-400F operating at 400 MHz) in dimethylsulfoxide (DMSO)-d 6 at 70 °C according to Van der Velden and Beulen [13].These characteristics were confirmed by 13 C NMR spectroscopy of the polymers solubilized in a 50/50 (v/v) D 2 O and deuterated acetone mixture.This technique gives in addition access to the average sequence lengths of vinyl acetate VAc 0 n defined by Moritani and Fujiwara [14].It may be noticed that sample PVA-73-685 has the highest VAc sequence length VAc 0 n .The SEC measurements were carried out with a Shimadzu LC-20AD liquid chromatograph equipped with two Varian PL gel 5 µm MIXED-C columns (column, injection and refractometer temperature: 30 °C; injection volume: 100 µL; solvent: tetrahydrofuran at 1 mL min −1 ) and a refractive index detector (Shimadzu RID-10A).The PVA samples were at first reacetylated as recommended by Bugada and Rudin [15] and the "universal calibration technique" with polystyrene standards was applied for the calculation of M n , M w and the polydispersity index PI = M w / M n .
Sodium dodecyl sulfate (SDS), obtained from Acros Organics with a purity of 99%, was used without any further purification.

Procedure for the Thermal Cloud Point Fractionation
The PVA solutions of 7.5 wt% were prepared by dissolving under agitation 7.50 g PVA powder in 92.50 g triple distilled and filtered (0.22 µm Millipore filter) water at room temperature for 24 hours.The obtained solutions were poured into a separating funnel and thermostated for 24 hours, the time required to reach an equilibrium situation corresponding to a constant fraction of phase separated coacervate at different fractionation temperatures.At the first fractionation temperature, 41 °C, two fractions were separated, a coacervate F 1 and a completely transparent supernatant layer F 1' .The coacervate F 1 was removed from the funnel and the remaining supernatant layer F 1' was used for a second fractionation step at 50 °C to give fractions F 2 and F 2' .The last two fractions, F 3 and F 4 , were separated from the supernatant fraction F 2' at 75 °C.
Each isolated fraction was characterized by NMR, SEC and DLS.

Sample Preparation
The PVA solutions of 1 wt%, for the DLS measurements, were prepared by diluting under agitation the required amounts of each separated fraction in triple distilled and filtered (0.22 µm Millipore filter) water at room temperature for 24 hours.
The SDS solutions were prepared by dilution of a 1 wt % "stock solution".To avoid the hydrolysis of SDS, all surfactant solutions were used within 24 hours.For the preparation of PVA/SDS solutions, each PVA fraction was directly diluted in the aqueous SDS solution at the required concentration under agitation for 24 hours at room temperature.

Dynamic Light Scattering
DLS measurements were carried out on a Malvern Nano-ZS6 Zetasizer equipped with a 4 mW He-Ne laser operating at a wavelength of 532 nm.The measurements were made at a scattering angle θ = 173° at a fixed temperature of 20 °C.Quartz cuvettes were used for all the experiments.Data (Dv and volume fraction) were acquired with the Malvern's Dispersion Technology Software version 4.20.
To determine the diameter of the particles, the data were collected in automatic mode, typically requiring a measurement duration of 70 seconds.The "data quality report" incorporated in the software indicated "good quality" for all the obtained data.For each experiment at a given temperature, the average of 5 consecutive measurements is indicated in the tables and figures.

Results and Discussion
These two PVA samples, PVA-73-650 and PVA-73-685, having very similar characteristics, such as DH , DPw but different VAc 0 n values, as shown in Table 1, were selected for our study in order to check that the "blockiness", e.g., the average VAc sequence length, could have an influence on the nanogel characteristics.
This aspect appears clearly in Figure 1, showing the particle size distribution of the initial PVA's at a concentration of 1 wt% in aqueous solution at 20 °C.As previously shown, peak 1 and peak 2 correspond to "free PVA chains" and to nanogels respectively [8].These so-called nanogels are formed by hydrophobic interactions between PVAc sequences as demonstrated by Lewis and Robinson [6] and by Aladjoff et al. [7].
The thermal cloud point fractionation process is schematically outlined in Figure 2: n -average sequence length of vinyl acetate and W i -weight %.
Three temperature steps above the cloud point are involved in this process.A first separation is carried out at 41 °C.After separation of this fraction F1, the temperature of the supernatant fraction F1' is increased to 50 °C.This operation leads to a coacervate F2 and a supernatant phase F2' which is further fractionated at 75 °C.The final fractions are F3 and F4 respectively.
The different fractions were characterized by NMR, SEC and by gravimetry.For a given PVA sample, PVA-73-650 and PVA-73-685 respectively, it can be noticed that: -the DH values of F 1 , F 2 and F 3 are lower than that of F 0 , the initial PVA sample; this is a direct evidence of a fractionation in composition; as expected, the VAc rich species, with a lower solubility, are predominant in the coacervate, -the DH values of F 4 , the most soluble fraction, are coming close to 80 mol%, -the DPw values of F 1 and F 2 are higher than that of F 0 , with afterwards a decrease for F 3 and F 4 ; this is indicative of a fractionation in molar mass, -the average VAc sequence length VAc 0 n has a tendency to decrease from F 1 to F 4 , -F 4 corresponds not only to fractions with lowest VAc content, but also to that of lower DPw and VAc 0 n respectively.
By taking into account the W i values of the fraction, given as the weight % with respect to F 0 , it is of interest to notice that the mass balance of the complete fractions process are of 93.7 and 95.3, for PVA-73-650 and PVA-73-685 respectively.A similar observation could be made for DH , DPw and VAc 0 n balances.In agreement with Zilberman et al. [10] and with Lerner and Alon [16] these results, obtained by thermal cloud point fractionation, are a direct evidence that the fractionation is influenced simultaneously by the polydispersity in composition, molar mass and sequence distribution of the VAc monomer units.
For a given PVA sample, the major differences in their characteristics, DH , DPw and VAc 0 n , which can be noticed between F , the most insoluble fraction, and F 4 the most soluble one, has a direct influence on their cloud points.In fact for sample PVA-73-650 the cloud point is shifted from 26.5 °C for fraction F 1 to 70.5 °C for fraction F 4 .For sample PVA-73-685 the corresponding shift is from 26.5 °C to 70.5 °C.Furthermore, for the fractions F 1 it is worth noting the 6% increase of W i for the sample PVA-73-685 with respect to PVA-73-650 with the VAc 0 n values of 4.5 and 4.0 respectively.The next step consisted in the determination by DLS of the particle size distributions and the volume fractions of the nanogels and the "free chains".An illustration of the bimodal size distribution of the non-fractionated sample PVA-73-650 (F 0 ) is given by Figure 3.This figure shows in addition the size distribution of the corresponding fractions F 1 and F 4 .
For F 1 it appears clearly, as compared to F 0 a size shift from 11.4 to 13.0 nm for the "free chains", which is in agreement with the DPw increase from 650 to 720.A similar trend is observed and for the nanogels, their size increase from 38.0 to 42.8 nm, with corresponding volume fraction of 18.0 and 24.2%.For fraction F 4 an almost monomodal size distribution curve can be noticed with an average particle size Dv = 8.1 nm.This decrease in size can directly be correlated to the low DPw of this fraction, e.g., 360 as compared to 720 for F 1 .If "free chains" are definitely predominant in fraction F 4 , a close inspection of the size distribution shows the presence of a small "tail" which might correspond to a minor residual amount of aggregates.
The detailed characteristics of the different fractions are given in Tables 2 and 3 for the sample PVA-73-650 and PVA-73-685 respectively.On further addition of SDS, such as a concentration of 5 wt%, the whole distribution curve is shifted to lower size values, corresponding to the "free chains"/SDS complex.For this fraction it can be admitted from DLS distribution curves that the nanogels are disaggregated at a SDS concentration around 5 wt% with respect to PVA.
The size characteristic of the different fractions of PVA-73-650 and PVA-73-685 at SDS concentrations of 1 and 5 wt% with respect to PVA are summarized in Table 4: From this table it is worth noting that for both PVA samples at a SDS concentration of 1 wt% with respect to PVA, the Dv values are decreasing from fraction F 1 to fraction F 4 .For these fractions, this tendency can be correlated to the corresponding decrease of vinyl acetate content and of VAc 0 n .However, it can be noticed that a SDS concentration of 1 wt% with respect to PVA is not sufficient to disaggregate completely the nanogels present in fractions F 1 , F 2 and F 3 .A complete disaggregation is only reached at higher SDS concentrations, such as 5 wt%, with a possible formation of "free chains"/SDS complex having a Dv value of around 7 nm.
A particular behavior can be observed for the most soluble fraction F 4 of lower DPw and VAc content.Moreover, this fraction does not contain nanogels, as shown in Tables 2 and 3, and the monomodal distribution can be considered as "free chains" with a Dv values between 8-9 nm.As no size change is observed in the presence of a SDS concentration of 1 wt% with respect to PVA, it can be concluded that the possible complex formation with SDS has no major influence on their size.
Further work is in progress in order to investigate the direct thermal cloud point fractionation of PVA/SDS complexes and preliminary tests have already shown that the cloud point of such systems are shifted to higher temperatures by addition of SDS.As a consequence the fractionation of "nanogels-free" systems would have to be performed at temperatures of around 80-85 °C.

Conclusions
In addition to various fractionation techniques, such as SEC and HPLC published up to now for PVA's, it could be confirmed that the thermal cloud point fractionation is a valuable alternative procedure to give an insight in the polydispersity characteristics of this type of polymer.
Two PVA samples, with same overall DPw and DH values but having different average sequence lengths of vinyl acetate VAc 0 n , were fractionated in three temperature steps between 40 and 75 °C.The nanogels with highest values of volume fraction and Dv were separated in the most insoluble fraction F 1 whereas the most soluble fraction F 4 no longer contains nanogels, only "free chains".
In the presence of an SDS concentration of 1 wt% with respect to PVA a partial disaggregation of nanogels present in fractions F 1 , F 2 and F 3 was noticed.A complete disaggregation is only reached at higher SDS concentrations, such as 5 wt%, with formation of "free chains"/SDS complex having a Dv value of around 7 nm.In the presence of SDS, no size change is observed for the fraction F 4 with higher DH values and lower "blockiness".

Figure 2 .
Figure 2. Schematic representation of the different fractionation steps.Molecular characteristics of the phases in equilibrium for sample PVA-73-650 and PVA-73-685 with DH -average degree of hydrolysis, DPw -weight average degree of polymerization, VAc 0

Table 2 .
Volume fraction and size average (Dv) of nanogels and "free chains" of different PVA-73-650 fractions at a concentration of a 1 wt%.

Table 3 .
Volume fraction and size average (Dv) of nanogels and "free chains" of different PVA-73-685 fractions at a concentration of a 1 wt%.

Table 4 .
The evolution of the size average (Dv) of different fractions of samples PVA-73-650 and PVA-73-685 at a SDS concentration of 1 and 5 wt% respectively with respect to PVA.