ARTICLE pubs.acs.org/Langmuir
Cosolvent Gel-like Materials from Partially Hydrolyzed Poly(vinyl acetate)s and Borax Lora V. Angelova,† Pierre Terech,‡ Irene Natali,§ Luigi Dei,§ Emiliano Carretti,§ and Richard G. Weiss*,† †
Department of Chemistry, Georgetown University, Washington, D.C. 20057-1227, United States SPrAM, UMR CEA/CNRS/UJF-Grenoble 1, INAC, Grenoble, F-38054, France § Department of Chemistry “Ugo Schiff” & CSGI Consortium, University of Florence, via della Lastruccia, 3 50019 Sesto Fiorentino, Italy ‡
bS Supporting Information ABSTRACT: A gel-like, high-viscosity polymeric dispersion (HVPD) based on cross-linked borate, partially hydrolyzed poly(vinyl acetate) (xPVAc, where x is the percent hydrolysis) is described. Unlike hydroHVPDs prepared from poly(vinyl alcohol) (PVA) and borate, the liquid portion of these materials can be composed of up to 75% of an organic cosolvent because of the influence of residual acetate groups on the polymer backbone. The effects of the degree of hydrolysis, molecular weight, polymer and cross-linker concentrations, and type and amount of organic cosolvent on the rheological and structural properties of the materials are investigated. The stability of the systems is explored through rheological and melting-range studies. 11B NMR and small-angle neutron scattering (SANS) are used to probe the structure of the dispersions. The addition of an organic liquid to the xPVAc-borate HVPDs results in a drastic increase in the number of cross-linked borate species as well as the agglomeration of the polymer into bundles. These effects result in an increase in the relaxation time and thermal stability of the networks. The ability to make xPVAc-borate HVPDs with very large amounts of and rather different organic liquids, with very different rheological properties that can be controlled easily, opens new possibilities for applications of PVAc-based dispersions.
’ INTRODUCTION Poly(vinyl alcohol) (PVA), produced industrially by the complete hydrolysis of poly(vinyl acetate) (PVAc), is known to form thermally reversible hydro-HVPDs in the presence of borate ions, which supply the cross-links between polymer chains needed for a 3D network. The properties of the hydro-HVPDs have been studied extensively by dynamic light scattering,18 rheology,5,912 small-angle neutron scattering,13,14 and 11B NMR.1518 We report that PVAcs with reduced (i.e., incomplete) hydrolysis fractions (xPVAc, where x is the percent of acetate groups that have been hydrolyzed) can be cross-linked with borate ions as well to form HVPDs containing large weight fractions of polar organic liquids. As will be shown, the macroscopic and microscopic properties of these networks are different from those employing PVA and water. Cosolvent incorporation into the PVAcborax systems increases their relaxation times and elastic character, and 11B NMR shows a large increase in the cross-linked borate species. The formation of cross-links between pairs of vicinal hydroxyl groups from two PVA chains and one borate ion (eq 1) is an exothermic process that leads to large increases in viscosity.12 The activation energy for breaking the four bonds constituting individual cross-links has been calculated to be 25 kJ/mol,12 and r 2011 American Chemical Society
the heat of cross-link formation is ca. 35 kJ/mol.3,15 The polymer network is dynamic because the cross-links are reversible; a steadystate concentration of cross-links is maintained under isothermal conditions. The nature of the cross-links proposed by Deuel and Neukom19 in eq 1 has been confirmed by 11B NMR investigations.15,19,20
In the first step of cross-link formation, a borate ion (commonly obtained by dissolving borax [Na2B4O7 3 10H2O] in water to form boric acid [B(OH)3] and borate ions [B(OH)4] that interchange rapidly under mildly basic solutions21) condenses Received: June 9, 2011 Revised: July 25, 2011 Published: August 17, 2011 11671
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Table 1. Summary of xPVAc Properties PVA
80PVAc_1
80PVAc_2
80PVAc_3
75PVAc
45PVAc
40PVAc
hydrolysis
>99%
80%
80%
80%
75%
45%
40%
Mw
∼31 600
∼21 100
∼35 600
∼47 300
∼7300
N/Ab
N/Ab
0.39
0.48
0.47
0.48
13.0
4.8
32.4
46.6
4.7
3.6
5.3
blockiness viscosity (mPa 3 s)a
a Information provided by the manufacturer. b Although Mw data for 45PVAc and 40PVAc have been obtained, they are not reported here because of concerns that the agglomeration (from aggregation) of these polymers may have resulted in erroneous results.
with a vicinal diol of one polymer chain to yield a monoborate ester and two molecules of water. Subsequent intra- or interchain reaction with another vicinal diol results in the formation of a didiol cross-link. Because several borate ions may attach to one chain, the polymers act as polyelectrolytes and separate from each other because of charge repulsion.22 Although electrostatic shielding effects from sodium counterions may allow some chains to contract and form cross-links by the esterification of a borate ion with pairs of vicinal diol units on different polymer chains,22 the contraction of the polymer chains destabilizes the cross-linked network. In effect, the conformation of the polymer chains in these HVPDs is the result of a balance among electrostatic repulsions, charge-shielding effects, and the polymer-chain excluded volume.6 PVAborax hydro-HVPDs exhibit both self-healing and flow over extended periods of time. The finite chemical relaxation time (i.e., the time required for an applied stress to decay to 1/e of its original value) at room temperature, 0.10.3 s,11,12 indicates that these dispersions are not solidlike or ideal dispersions whose relaxation time approaches infinity. In fact, the finite relaxation times of PVAborax hydro-HVPDs indicate that they are viscoelastic dispersions but not true gels;11 rheologically, they behave like elastic solids at short observation times (high frequencies) and like viscous liquids at long observation times (low frequencies). The short relaxation time and small amount of energy required to break and reform the cross-links account for the well-known “slime” consistency of these materials. To the best of our knowledge, the borate-initiated “gelation” of xPVAc with x < 80 (i.e., less than ∼80% hydrolysis) has not been reported.3,5,9,11,23,24 Investigations of PVAborax HVPDs with mixed liquids have focused mostly on DMSOwater systems25,26 and two studies with HVPDs prepared from (g80) PVAc by our group.27,28 Here, we report the preparation and properties of temporally stable, transparent, translucent HVPDs containing xPVAcborax networks that can include up to 75 wt % of a polar organic liquid as a water cosolvent. Insights into the structural and viscoelastic changes that occur when the concentration or nature of the HVPD components are changed have been obtained from analyses of data from rheological, thermal, and spectroscopic (NB, 11B NMR) measurements. In addition to providing fundamental insight into factors controlling the efficiency of the cross-links and the interactions between the liquid and network components of the HVPDs, these studies describe a new type of soft matter with a very broad range of properties that can be tuned to meet the requirements of specific applications. Related systems have already been used in art conservation,28 and the low-hydrolysis PVAcborax HVPDs described here show great potential for use in this field as well.
’ EXPERIMENTAL SECTION Materials. Partially hydrolyzed poly(vinyl acetate)s, as random copolymers, were supplied by Kuraray Co., Ltd.; the polymers are
described in Table 1. They were washed copiously with ice-cold water and dried under vacuum in order to eliminate byproducts and residual free acetate; see Figures S1S3 in the Supporting Information for the 1H NMR spectra of the polymers after purification. Hydrolysis fractions were determined by integrating the acetate methyl and main-chain methylene 1H NMR signals (Supporting Information Figure S4). Sodium tetraborate decahydrate (>99.5%, Fluka) was used as received. Methanol (Fisher, HPLC grade), ethanol (Aldrich, anhydrous, denatured), 1-propanol (Fisher, certified normal grade), 2-propanol (HPLC grade, 99.9%, Fisher), 1-pentanol (Aldrich, 99+%), methyl acetate (99.8%, Fluka), acetone (histological grade, Fisher), 1-methyl2-pyrrolidinone (extra dry, with molecular sieves, water 99% as indicated in Table 1 by their acronyms. The lowest polymer and cross-linker concentrations necessary for the formation of stable HVPDs were investigated systematically by the falling drop method (i.e., visual observation of the lowest temperatures required to induce flow when a sample was inverted in a sealed tube in a heated water bath).36 The amount of one of the components of the HVPDs was varied incrementally while keeping all others constant (Figure 1). The liquid composition was chosen to make the dissolution of polymers with a large acetate content favorable. The most thermally stable HVPDs with a 30/70 1-propanol/water liquid composition were obtained when the ratio of polymer hydroxyl groups to borate ions (OH/B(OH)4) was in the range of 5/1 to 20/1 (Table 2). These ratios were maintained throughout many of the following 11673
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Figure 1. Melting ranges (vertical lines) for HVPDs with 30/70 1-propanol/water mixtures: (red f) PVA, (b) 80PVA_3, (orange 2) 75PVAc, (blue 1) and 45PVAc, (green 9) 40PVAc. (A) Varying amounts of polymer ([borax] = 2.0 wt %). (B) Varying amounts of borax at 11 wt % 40PVAc, 16 wt % 45PVAc, 5 wt % 75PVAc, 4 wt % 80PVA_3, and 4 wt % PVA.
studies even though the types and amounts of organic liquids were varied; the optimization of thermal stability was not determined at other liquid compositions. The threshold concentrations of the five polymers at the onset of 3D network formation were deduced from the large increase in the intrinsic viscosity observed at given polymer concentrations (Supporting Information Figure S7). The values obtained from the η0 plots are described in Table 2 (intrinsic viscosity). Although the values of η0 at low polymer concentrations are affected by a large imprecision, the trend is correct and the ranges of values do not alter the overall conclusions. These concentrations are much lower than those suggested by melting-range experiments that were performed on HVPDs prepared with 30/70 1-propanol/water. The two types of experiments measure different aspects of the system; the melting range tests determine the thermal stability of the networks, and the intrinsic viscosity indicates the lowest necessary amounts of the two components for network formation. The two different polymer and borax concentrations used in Table 2 and obtained from the two experimental methods can be viewed as the minimum concentrations required to form a network in water (from η0 values) and the minimum required to make a stable HVPD system at room temperature that is also thermally stable to at least 50 C (melting-range value). The polymer weight fraction needed to prepare the HVPDs was found to depend on the molecular weight of the xPVAc as well as the value of x. Polymers with fewer hydroxyl groups (i.e., smaller x values) require larger concentrations for network
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formation. However, in 30/70 1-propanol/water and with 2 wt % borax, high molecular weights can compensate in part for low hydrolysis fractions. For example, only 3 to 4 wt % 80PVA_3 (the highest Mw polymer of the set; Mw ≈ 47 300) was required for stable HVPD formation. In pure water, only 1.3 wt % of this polymer is required for the formation of a 3D network whereas 3.5 wt % of 75PVAc (Mw = 7300) is needed. As will be shown by 11 B NMR spectroscopy and rheology, high molecular weight in the polymers contributes greatly to the dispersion strength but not through increased cross-linking. In fact, increasing molecular weight results in a reduction of the cross-linked boron species in these systems. The high thermal stability and elasticity of HVPDs prepared from high-Mw polymers must therefore be a result of increased hydrogen bonding and chain entanglements. The melting-range results show that the thermal stability of the HVPD systems increases more drastically when the polymer concentration is increased. The change in thermal stability due to cross-linker concentration is not as pronounced. These results are in agreement with neutron spinecho results obtained for PVA/borax systems by Kanaya et al. showing that cross-linking is not necessary for HVPD formation whereas chain entanglements are crucial.25 Kjøniksen et al. draw similar conclusions from investigations of the viscoelasticity of PVAglutaraldehyde systems.10 The dispersion strength of the PVA-based hydroHVPDs increases with increasing concentrations of glutaraldehyde, an irreversible cross-linking agent.10 Increasing chain entanglements and more hydrogen bonding among polymer chains (from increased polymer concentrations) and more crosslinks (from higher cross-linker concentrations) also increase the HVPD strength. These effects are more noticeable at low polymer concentrations, where the network is more fragile and sensitive to perturbations.10 Our results are determined by, among other factors, the fraction of hydroxyl groups that exist as vicinal diols because they are the only ones that can effectively participate in cross-links. Nevertheless, even when the availability of hydroxyl groups is decreased because of a lower degree of hydrolysis, the polymer concentration still seems to be the dominant factor in determining the thermal stability of the borax cross-linked system. Incorporation of Organic Liquids. The presence of some acetate groups on the polymer chains increases the ability of larger fractions of polar organic liquids to be incorporated into xPVAcborax HVPDs as compared to the PVAborax systems. Although some water is necessary to solubilize the borax component as the HVPDs are being formed, the maximum volume fraction of organic liquid possible is roughly inversely proportional to the degree of hydrolysis of the polymers (Table 2). The obtained dispersions are transparent or translucent and are stable over the course of weeks in closed containers. Some of the less polar liquids, such as 1-pentanol and ethyl acetate, can be incorporated, but the resulting HVPDs undergo syneresis within minutes to hours. Upon the basis of the density of the expelled liquid from a 1/1 1-pentanol/water system, it is almost exclusively 1-pentanol (Supporting Information Table S1). Approximately 25% of the 1-pentanol is retained by HVPD, resulting in an opaque appearance. Effects of Organic Solvents on Thermal Stability. Melting range tests on HVPDs composed of 40PVAc, 75PVAc, 80PVAc_3, and PVA with varying amounts of organic liquids in the solvent portion show a general increase in stiffness with increasing organic content (Supporting Information Figure S8); they become less elastic and more brittle. HVPDs composed of 11674
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Figure 2. (A) 1.0 wt % borax in D2O. (B) 4 wt % 75PVAc and 1.0 wt % borax in D2O (11/1 molar excess of polymer OH groups with respect to borate ions).
80PVAc_3 and borax at the examined concentrations generally melt above the boiling points of the incorporated organic liquids for the concentrations examined (Supporting Information Figure S8C); the observed decrease in the melting-range temperatures of these HVPDs may not reflect their actual thermal stability. 11 B NMR Studies. The influence of the organic liquid content on the structure of the HVPDs was quantified by 11B NMR spectroscopy, which can be used to differentiate between the free and cross-linked boron species in the systems.9 As in NMR of other nuclei, the chemical shift of boron depends on its environment. The broad nature of the peaks is a result of the quadrupole moment of the 11B nucleus, which offers an alternative relaxation pathway (i.e., one of the primary factors that influence the peak width at half-height is the inverse of the quadrupolar relaxation time).38 The broad peak from boron atoms not involved in crosslinks appears at a chemical shift that depends on the absolute concentration and the relative concentrations of the exchanging boric acid (19.3 ppm) and the borate ion (1.6 ppm) whereas the cross-linked borate species has a chemical shift of approximately 1.5 ppm (Figure 2).38 The addition of polymer to aqueous borax results in a downfield shift in the free boron peak; some of the borate ions are being held in cross-links, and the diffusion allowing for exchange between boric acid and borate ions is slowed in highly viscous HVPD.37 Although the molar concentration of hydroxyl groups in the polymer solutions is much higher than the concentration of borate ions in the studied HVPDs, only a fraction of the borates participate in cross-link formation.12,25 The actual amount can be estimated from the integration of the appropriate peaks in the 11B NMR spectra of the HVPDs. For example, only 6% of the borates present in a 4 wt % 75PVAc/1 wt % borax in D2O sample participate in cross-links (Figure 2). If the concentrations of the polymer and borax are increased incrementally while keeping the w/w ratios constant, then the quantity of borate species that are cross-linked remains relatively constant with only a small increase in the absolute concentration of cross-linked borate ions (Supporting Information Figure S9).27 Increasing the borax concentration while keeping the acetone/ water content at 30/70 and the polymer concentration at 4 wt %
Figure 3. Concentrations and fractions of borate ion participating in cross-linking (of total boron species) in HVPDs composed of 4 wt % 75PVAc with 30/70 acetone/water liquid as calculated from the 11B NMR spectra.
results in an apparent decrease in the fraction of boron species participating in cross-links although the absolute concentration of cross-links increases (Figure 3). The increase in the absolute number of borate ions participating in cross-links can be correlated with the increase in the thermal stability of the HVPDs with increasing borax concentrations (Figure 1B). The increase in thermal stability found when organic liquids were incorporated into the systems can be attributed to increases in the cross-links. The fraction of cross-linked borate ions increases from 7 to 45% when the liquid is changed from D2O to 50/50 acetone-d6/D2O (Figure 4). The free boron peak also shifts downfield as the acetone concentration is increased, indicating that the concentration of boric acid is higher than that of the borate ion. 11 B NMR can also be used to investigate the effects of molecular weight on the cross-linking of the materials. HVPDs prepared with high-molecular-weight PVAcs require a lower polymer concentration and have higher thermal stabilities (Figures 1 and S8C). These systems also have a higher degree of elastic character (vide infra). Three polymers (80PVAc_1, 80PVAc_2, and 80PVAc_3) with approximately the same degree 11675
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Figure 4. (A) 11B NMR spectra of (red 9) 4 wt % 75PVAc and 1 wt % borax in D2O, (green 9) 4 wt % 75PVAc and 1 wt % borax in 30/70 acetoned6/D2O, and (blue 9) 4 wt % 75PVAc and 1 wt % borax in 50/50 acetone-d6/D2O. (B) Graph of the percentage of boron atoms involved in cross-links (with respect to the total number of boron species) as calculated from plot A.
Figure 5. (A) 11B NMR of hydro-HVPDs composed of 4 wt % 80PVAc and 1 wt % borax from 80PVA_1 (green —), 80PVAc_2 (red —), and 80PVAc_3 (blue —). (B) Percentage of boron atoms involved in cross-links (with respect to the total number of boron species) as calculated from three separate samples, one of which is shown in plot A; the color code is the same as in plot A.
of hydrolysis and structure (Supporting Information Figure S10) were used to study the effects of molecular weight on borate cross-linking (Table 1). 11B NMR spectra of the hydro-HVPDs prepared from these three polymers show a significant decrease in the amount of cross-linked boron with increasing molecular weight (Figure 5). At low concentrations (1 wt % polymer and 0.25 wt % borax), the number of cross-links in HVPDs prepared from each polymer is relatively small and constant (Supporting Information Figure S11). Although there is a large change in the proportion of borate ions participating in cross-linking with increasing 1-propanol content in the liquid portion of HVPDs composed of 80PVAc_1 at 4 wt %, this change is present only in samples prepared from 80PVAc_3 when the polymer concentration is kept sufficiently low at 2 wt % (Figure 6). When the 1-propanol proportion in the liquid component of the HVPDs is varied in systems of 80PVAc_3 with a polymer concentration of 4 wt %, there is a significant attenuation of the effect of the organic solvent on the increase in borate cross-linking (Figure 6). Effects of Organic Solvents Investigated by SANS. The topography of a molecular gel network can be very sensitive to its
Figure 6. Complexed borate ion (with respect to the total number of boron species) in HVPDs composed of (9) 4 wt % 80PVAc_1 and 1 wt % borax, (red b) 4 wt % 80PVAc_3 and 1 wt % borax, and (blue 2) 2 wt % 80PVAc_3 and 0.5 wt % borax as calculated from 11B NMR spectral integrations.
preparation protocol. Here, the large fraction of borate ions ([75PVAc]/[borax] = 0.25 or OH/[B(OH)4] = 11/1) is one 11676
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Langmuir of the remarkable features of these freshly prepared (lifetime ≈ 2 to 3 days) systems. It is therefore expected that a large fraction of cross-links develops in the HVPDs. Such cross-links have been previously characterized13 as small crystallites of a few tenths of an angstroms in size. The partially hydrolyzed polymer chains with only ca.75 mol % OH groups are normally subject to both inter- and intramolecular hydrogen bonds, and thus the degree of swelling in such a network is expected to be different from that for fully hydrolyzed systems.13,39 The network mesh size represents the mean distance between consecutive cross-linking points and provides a measure of the porosity of the network. The more homogeneously a PVA network is swollen, the more dispersed are the constitutive PVA chains and cross-links. As a consequence, the mesh size of PVA networks is expected to increase with an increasing degree of swelling. Using this general view of a PVA network, we have investigated the capacity of the topography of the PVAcborax hydro-HVPDs to be modified by the interaction parameter between PVAc chains and mixtures of water and acetone as the solvent. As classically described by the FloryHuggins solution theory,4042 a polymer solvent interaction parameter χ12 is defined for a given solvent solute couple. Its variation with composition and temperature can be evaluated by vapor-pressure measurements43 and can be complex. Although χ12 = 0.494 (at T = 30 C) can be estimated for PVA in water,44 no information is available for PVAc in acetone. Nevertheless, it is reasonable to consider acetone to be a “bad” solvent with respect to the solubility properties. These preliminary considerations rule out the aggregation of polymer chains in polyelectrolyte salt-free solutions that result45 from the sensitive interplay between electrostatics and short-range monomer monomer interactions.
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It is known that the potential of mean force between particles can be tuned from repulsive to attractive by varying the solvent quality.46 The magnitude and sign of this potential can enormously influence the heterogeneity and time stability of HVPDs. On the basis of the excess chemical potential of mixing, polymersolvent interactions are energetically favorable in a good solvent (leading to chain extensions) whereas polymerpolymer interactions are preferred in a poor solvent (leading to more chain coiling). Thus, increasing the proportion of acetone in a hydro-HVPD composed of an acrylamide cross-linked with bisacrylamide has been used to trigger the swollen-to-shrunken gel transition.47 Here, the SANS curves show striking differences when the acetone content is varied in the 75PVAcborax HVPDs. Figure 7A shows that a broad and intense hump is present at Q ≈ 0.024 Å1 in the hydro-HVPD whereas it moves to lower Q in the 30/70 acetone/water HVPDs and vanishes in 50/50 acetone/water HVPD. The broad peak appearing in the hydro-HVPD can be assigned to long -distance correlations in the network of PVAc chains. Given the high concentration of the borax cross-linking agent, it is reasonable to speculate that the number and/or size of the cross-linked domains should be high and responsible for the specific neutron-scattering correlation peak. In addition to the cross-links, the 75PVAc chains (or bundles of chains) contribute to the scattering. As mentioned above, it is assumed that an increase in acetone content decreases the solvent quality of the mixture. As a consequence, more 75PVAc chains are associated in polymer-rich domains (with or without the borax cross-links), and at constant 75PVAc concentration, the average spacing between the domains increases. The contribution of these
Figure 7. SANS of 4 wt % 75PVAc and 1 wt % borax HVPDs. (A) Overlay of three HVPDs investigated with different liquid compositions: hydroHVPD (0.0, green), 30/70 acetone/water (0.3, blue), and 50/50 acetone/water (0.5, red). (B) PY modeling of the correlation peak of hydro-HVPD. (C, D) OZ modeling of 30/70 acetone/water and 50/50 acetone/water HVPDs, respectively. Blue dots are experimental data, and red dots are the experimental data used in the fitting procedures. 11677
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Figure 8. Angular frequency sweeps of HVPDs taken in the linear viscoelastic regions (strain = 1%) showing (9) G0 and (b) G00 . (A) 6 wt % 75PVAc and 1.4 wt % borax with varying amounts of 1-propanol: (9) 0, (blue 9) 30%, and (green 9) 50%. (B) G0 plateau (taken as Pa at 79.2 rad/s) as a function of the 1-propanol concentration. (C) 6 wt % 75PVAc and 30/70 1-propanol/water with varying amounts of borax: (9) 0.5 wt %, (red 9) 1.0 wt %, (orange 9) 1.5 wt %, (green 9) 2.0 wt %, (blue 9) 2.5 wt %, and (purple 9) 3.0 wt %. (D) G0 plateau (taken as Pa at 79.2 rad/s) as a function of the concentration of borax. Data are obtained from three trials, and one standard deviation is shown in the graphs.
domains to the scattering is not found in the low-Q part of the scattering, where only a sharp decay is detectable as a tail of the scattering by the cross-linked chains (curve 0.3 in Figure 7A). This feature precedes a smooth decay with a convex profile attributed to the swollen part of the PVA chains. It is reasonable to consider this contribution as arising from dynamic fluctuations characterized by a correlation length ξ and following eq 2. SðQ Þ ¼
Sð0Þ 1 þ Q 2 ξ2
ð2Þ
In this context, the correlation peak observed in the hydroHVPD can be considered to be the static part inhomogeneity created by the cross-links (see analysis below) whereas the smooth decay is the dynamic component represented by the OrnsteinZernike (OZ) function (eq 2).39,48,49 The formalism deals with equilibrated solutions of polymers in solution in a good solvent. The structure factor function of the concentration fluctuations at low Q with a correlation length ξ proposed in eq 2 may be specified in that the prefactor, S(0), is related to the crosslink density and osmotic modulus. Figure 7C,D shows the agreements of the OZ formalism applied to appropriate parts of the measured scattering in 75PVAcborax HVPDs with 30/70 and 50/50 acetone/water compositions. Increasing the acetone volume fraction from 30 to 50% clearly increases the amplitude of the prefactor from 4.80 to 49.5 and thus (expectedly) follows the changes in the mechanical properties of the network. Simultaneously, the correlation length increases
by a factor of 2.7 from 58.9 to 158. As the solvent quality decreases, the cross-links tend to become more concentrated in select regions, thereby increasing the average distance between such regions. The main scattering feature of the hydro-HVPD is the correlation peak observed at Q ≈ 0.024 Å1 (Figure 7B). In the aqueous borax solutions of PVA, large increases in the viscosity can be assigned to the cross-linking of 75PVAc chains through H bonds between OH groups on 75PVAc chains and borate ions. With the 50/50 acetone/water system (Figure 7A), the broad correlation peak that was observed in the hydro-HVPD is no longer visible or even distinguishable by its low-Q tail as in the 30/70 acetone/water system. At 50/50 acetone/water, the scattering is overwhelmed by the signal from the more or less swollen bundles of 75PVAc chains (for which OZ has been applied; see above) connecting the polymer-rich regions. As frequently observed in gels, the polymer-rich regions can be considered to be randomly dispersed heterogeneities and can be accounted for by the DebyeBueche (DB) model. The DB framework assumes that spatial correlations of average length Ξ are attenuated according to an exponential correlation function g(r) = exp(r/Ξ) so that the intensity can be expressed by eq 3. I ¼
Ξ3 ð1 þ Q 2 Ξ2 Þ2
ð3Þ
Consequently, the main scattering feature should be a Q4 decay, but none is observed in our systems; the DB model cannot 11678
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Figure 9. Crossover frequencies for HVPDs composed of (A) 4 wt % PVA and 1.1 wt % borax, (B) 4 wt % 80PVAc_3 and 1 wt % borax, (C) 6 wt % 75PVAc and 1.4 wt % borax, and (D) 11 wt % 40PVAc and 2 wt % borax with varying amounts of (9) 1-propanol, (red b) ethanol, or (blue 2) Nmethylpyrrolidone. Results shown are from one angular frequency sweep; the reproducibility of the results is shown in Supporting Information Figure S12.
the radial polydispersity of the spheres, a normalized Gaussian function can be used as a convolution with the intensity expression. IðQ Þ≈SðQ ÞÆjFðQ Þj2 æ FðQ Þ ¼
Figure 10. Angular frequency sweeps and (9) G0 and (b) G00 crossover frequencies of hydro-HVPDs composed of 4 wt % PVA and 1 wt % borax from (black symbols) 80PVAc_1, (red symbols) 80PVAc_2, and (blue symbols) 80PVAc_3. The strain is 1% in the linear viscoelastic region for all three.
account for the correlation peak appearance in the scattering curve. In other words, the representation of the network as a fractal packing of uncorrelated dense clusters is not appropriate. Therefore, the heterogeneities are somewhat ordered and not randomly dispersed in a biphasic system. In an alternative model, the cross-links can be considered to be polydisperse spheres interacting via a hard-sphere potential. In a simplified scattering description, the intensity results from eq 4, where F(Q) is the formfactor function for homogeneous spheres (radius R) and S(Q) is their corresponding structure factor (eq 5). To take into account
ð4Þ
3 sinðQRÞ QR cosðQRÞ ðQRÞ3
ð5Þ
S(Q) can be evaluated using the analytical form of the Percus Yevick (PY) approximation for hard-sphere repulsions.50 The PY equation is commonly used as a convenient method to access the radial distribution function in interacting systems of soft matter. " 1 24ϕ ¼ 1 þ 3 aðsin x x cos xÞ SðQ Þ x þ
2 2 1 x cos x þ 2 sin x x2 x
2 ## ϕa424 6 12 24 þ þ 4 1 2 sin x 1 2 þ 4 x cos x 2 x3 x x u ð6Þ 4
with x = 2QD, a = (1 + 2ϕ) (1 ϕ) , and b = /2ϕ(ϕ + 2)2(1 ϕ)4 The best fit using eq 6 with data of the hydro-HVPD in deuterated water is shown in Figure 7D. D can be considered to be the distance of closest approach of spheres of radius R and 2
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Figure 11. (A) Free water indexes derived from the normalized ΔHexp of HVPDs composed of 11 wt % 40PVAc and 1 wt % borax with varying amounts of 1-propanol in the liquid component. (B) Free water indexes derived from the ΔHexp of HVPDs composed of 11 wt % 40PVAc and 75/25 2-propanol/water with varying concentrations of borax. DSC thermograms are available in Supporting Information Figure S14.
radial polydispersity ε = ΔR/R. According to the PY modeling, cross-links are spheres of 50 Å radii interacting with a mean correlation distance of 120 Å and a volume fraction of fcross-links ≈ 0.05. As expected, the cross-link domains are very polydisperse (as indicated by the large value of ε). Interestingly, the center-tocenter distance between the cross-links, 220 Å, is near a rough correlation distance extracted simply from 2π/0.024 ≈ 260 Å. At this stage, it is not necessary to consider other refining models, such as that of TeubnerStrey51 or Beaucage,52 to describe more ordered structures. Although SANS has been used frequently to characterize the structures of noncovalently and covalently linked polymer gels and, in particular, PVA hydroHVPDs,14,25,39,5355 this is the first study demonstrating the effect of solvent quality on both static and dynamic heterogeneities in a network. In that regard, Kanaya et al.25 analyzed the neutron spinecho profiles of three types of PVA dispersions: one dispersed in a mixture of deuterated dimethyl sulfoxide and water, another in a borax solution, and a third that was chemically cross-linked. The importance of the nature of the cross-link domains in the HVPD dynamics was demonstrated by the very different behavior of the three systems. In the case of the borax HVPD, it was shown that the dynamics of the chains were unaltered by the specific HVPD structure and they could be fitted well by the classical Zimm model. This observation is consistent56 with our present description of the smooth decay characteristics of the dynamic entanglements of Gaussian chains in a good solvent according to the OZ function.57 Effects of Organic Solvents on Rheological Parameters. The rheological properties of the HVPDs show that the incorporation of organic liquids increases their relaxation times, and they can be increased further when polymers with larger molecular weights or specific organic liquids are employed. The relaxation time here is taken as approximately the inverse of the storage modulus (G0 ) and the loss modulus (G00 ) crossover frequencies.58 The enhanced elastic character of the system resulting from an increased borax concentration in HVPDs with 30/70 propanol fractions is similar to that observed when the liquid composition is altered (Figure 8). There may be a rough correlation between the relaxation time and the polarity of the liquid. The relaxation times increase in the order N-methylpyrrolidone > ethanol >1-propanol when the other component concentrations are kept constant. Although this trend is most evident in the crossover frequencies of the 40PVAc systems (Figure 9D), further studies must be performed to confirm its origin.
The effect of molecular weight on the elastic character of the HVPDs was studied directly using dispersions prepared from 80PVAc_1, 80PVAc_2, and 80PVAc_3 (Figure 10). As expected, the relaxation times of HVPDs prepared from 80PVAc_3 are much longer than those of HVPDs prepared from the lowerMw polymers. Also, extensional rheology studies show that HVPDs prepared from higher-M w polymers are stronger (Supporting Information Figure S13) than those made from lower-M w polymers. Effect of Added Organic Solvents on the Free Water Index. The changes induced by increased weight fractions of organic liquids have been examined by differential scanning calorimetry (DSC) using the free water index (FWI) (eq 7), a measure of the free water (i.e., freezable at reasonably low temperatures) in a system.58,59 FWI ¼
ΔHexp AΔHinit
ð7Þ
In this expression, ΔHexp is the enthalpy change (in J g1 of HVPD) determined from a DSC heating endotherm, A is the weight fraction of water in the dispersion, and ΔHinit is the specific enthalpy of the fusion of the water in the HVPD if it were composed of completely freezable water at 0 C.29 As the organic content of the liquid in the HVPDs is increased, the FWI decreases dramatically (Figure 11A). This decrease is an indicator of increased structuring of the remaining water caused by the presence of the organic liquid.28 Because the aqueous portion of the liquid in these systems is expected to reside closer on average to the polymers (bound and nonfreezable water), the change in FWI can be attributed to increased cross-linking. By contrast, the concentration of borax does not have a large influence on the FWI, at least within the limited range investigated (Figure 11B). As noted previously, the major changes caused by increasing the borax concentration appears to involve its influence on the cross-linking density.
’ CONCLUSIONS The thermal and rheological properties of HVPDs made from partially hydrolyzed poly(vinyl acetate), borax, water, and an organic liquid can be varied over a very wide range by changing the degree of hydrolysis and molecular weight of the polymer and the concentrations of the components as well as the type and amount of organic liquid. It has been shown that increasing concentrations of PVAc and borax, as well as increasing 11680
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Langmuir molecular weights of the polymers, result in higher thermal stability and behavior closer to that of true gels. Chain entanglements and hydrogen bonding among polymer hydroxyl groups are probably the greatest contributors to the increase in network strength when the polymer concentration or molecular weight is varied. As shown by 11B NMR, the absolute concentration of cross-links increases when higher concentrations of borax are used in the preparation of the dispersions. The increases in the thermal stability and relaxation time of the HVPDs containing large fractions of organic liquids have been shown to be related to increases in the degree of cross-linking. These effects may be a result of the interplay among several phenomena, including the following: (1) the borate ions are insoluble in organic liquids and prefer to be in the cross-linked state or in the boric acid form and (2) in dispersions prepared from polymers with high degrees of hydrolysis (i.e., more PVAlike), the polymer chains tend to become increasingly folded in the presence of most organic liquids; the proximity of hydroxyl groups on adjacent chains encourages cross-linking. The migration of borate ions closer to the polymer chains, accompanied by their sodium counterions, results in a decrease in the fraction of water in the system that is free. As shown by SANS measurements, the polymer chains tend to form bundles with concentrated cross-links in poor organic solvents. It is interesting that the increase in cross-linking from the addition of an organic liquid is less in dispersions prepared from 80PVAc with a high molecular weight (N. B., 80PVAc_3) even though the thermal strength and elastic strength of the HVPDs are higher than those of HVPDs prepared with the same weight concentrations of lower-molecular-weight 80PVAc_1 and 80PVAc_2 polymers. The attachment of borate ions to multiple sites on a polymer chain results in a polyelectrolyte-like species in which the cross-linking is reduced. This reduction can be attributed to several factors. Two which seem probable and might work in conjunction with each other are local changes in ionic strength, especially near the cross-link points, and the increased proximity of the negatively charged borate ions, which causes electrostatically unfavorable interactions. Because the three polymers are 80% hydrolyzed, their behavior should be closer to that of PVA than that of PVAc, and because 1-propanol is not a good solvent for PVA, polymer-chain folding should be more favorable. The ancillary increased proximity of vicinal diol groups on either the same or different polymer chains then increases the probability that borate cross-links will form. This effect is especially noticeable when the concentration or molecular weight of the xPVAc is low, so that chain entanglements and interchain H-bonding interactions are less important. The diversity of the systems discussed here opens the possibility for new applications of these materials. The data provided are a blueprint for others to design organo/aqueous highviscosity polymeric dispersions based on xPVAc polymers and borate cross-links with specific structural and rheological properties. One of our future applications of these dispersions will be for art conservation, preservation, and cleaning of cultural heritage works.2830,59,60
’ ASSOCIATED CONTENT
bS
Supporting Information. Equation for the calculation of the degree of hydrolysis. 1H NMR spectra of 75PVAc in DMSO-d6 showing relevant peak assignments. 1H NMR spectra of polymers in DMSO-d6. 13C NMR spectra of polymers. 1H NMR
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spectrum of 75PVAc in DMSO-d6 showing integrated areas of methylene and acetate hydrogens. Ratios of complexed to free borate species at different pulse observation times. Strain sweep performed on a HVPD composed of 6 wt % 75PVAc and 1.4 wt % borax with 30/70 1-propanol/water.Melting temperature ranges of HVPDs. Plots of η0 versus xPVAc concentration in H2O. Concentration and percentage of borate ions participating in crosslinks. 1 H NMR spectra of 80PVAc_1, 80PVAc_2, and 80PVAc_3 in DMSO-d6. Percentage of boron species as crosslinking borate ions in hydro HVPDs. Crossover frequencies for HVPDs. Extensional rheology of hydrogels and the maximum normal force found for each HVPD. Heating DSC thermograms of 40PVAc and borax HVPDs. Weight and density of liquid expelled from HVPDs. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This article is dedicated to Prof. Kailasam Venkatesan on the occasion of his 80th (by Indian count) and 79th (by western count) birthday. L.V.A. and R.G.W. thank the National Science Foundation for its support of this research, Drs. Gerald O. Brown and Patricia M. Cotts of DuPont for performing the GPC experiments, and Mr. Ken Kumake of the Kuraray Co., Ltd. for providing the polymer samples. We are especially indebted to Dr. Barbara Berrie for her careful and critical reading of the text. I.N. and L.D. thank the Tuscany Region, Italy, TemArt Project European Fund for Regional Development (POR CreO FESR 2007-2013) for financial support. L.V.A. thanks the Clare Boothe Luce Program for a scholarship. We acknowledge the support of the National Institute of Standards and Technology, U.S. Department of Commerce, in providing the neutron research facilities used in this work. ’ REFERENCES (1) Koike, A.; Nemoto, N.; Inoue, T.; Osaki, K. Macromolecules 1995, 28, 2339–2344. (2) Nemoto, N.; Koike, A.; Osaki, K. Macromolecules 1996, 29, 1445–1451. (3) Koga, K.; Takada, A.; Nemoto, N. Macromolecules 1999, 32, 8872–8879. (4) Takada, A.; Nishimura, M.; Koike, A.; Nemoto, N. Macromolecules 1998, 31, 436–443. (5) Inoue, T.; Osaki, K. Rheol. Acta 1993, 32, 550–555. (6) Lin, H. L.; Liu, Y. F.; Yu, T. L.; Liu, W. H.; Rwei, S. P. Polymer 2005, 46, 5541–5549. (7) Chen, C. Y.; Yu, T. L. Polymer 1997, 38, 2019–2025. (8) Chen, C. Y.; Guo, J. Y.; Yu, T. L.; Wu, S. C. J. Polym. Res. 1998, 5, 67–76. (9) Maerker, J. M.; Sinton, S. W. J. Rheol. 1986, 30, 77–99. (10) Kjoniksen, A. L.; Nystrom, B. Macromolecules 1996, 29, 5215– 5222. (11) Robb, I. D.; Smeulders, J. B. A. F. Polymer 1997, 38, 2165–2169. (12) Schultz, R. K.; Myers, R. R. Macromolecules 1969, 2, 281–285. (13) Shibayama, M.; Kurokawa, H.; Nomura, S.; Muthukumar, M.; Stein, R. S.; Roy, S. Polymer 1992, 33, 2883–2890. (14) Wu, W. L.; Kurokawa, H.; Roy, S.; Stein, R. S. Macromolecules 1991, 24, 4328–4333. (15) Sinton, S. W. Macromolecules 1987, 20, 2430–2441. 11681
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