Does Plasticizer Penetrate Tightly Bound Polymer ... - ACS Publications

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Does Plasticizer Penetrate Tightly Bound Polymer in Adsorbed Poly(vinyl acetate) on Silica? Boonta Hetayothin,†,§ Roy A. Cabaniss,‡ and Frank D. Blum*,∥ †

Department of Chemistry and Materials Research Center and ‡Department of Computer Science, Missouri University of Science and Technology, Rolla, Missouri 65409, United States § Department of Chemistry and Biochemistry, California State University, Los Angeles, California 90032, United States ∥ Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078, United States S Supporting Information *

ABSTRACT: When only tightly bound poly(vinyl acetate) (PVAc) was adsorbed on silica, the plasticizer, di(propylene glycol) dibenzoate (DPGDB), was not effective in lowering its glass transition. To determine why the plasticizer was not effective, we probed the behavior of the deuterated plasticizer di(propylene glycol) dibenzoate (DPGDB-d10)/PVAc/silica system using deuterium (2H) solid state NMR and temperature-modulated differential scanning calorimetry (TMDSC). PVAc with 37% (w/ w) of plasticizer/polymer was adsorbed on silica in adsorbed amounts of 2.60 and 0.76 mg PVAc/m2 silica. The dynamics of the plasticizer in the adsorbed PVAc was found to be more motionally heterogeneous than that observed in bulk samples. The NMR results provided firm evidence that when there is only a small amount of adsorbed polymer (e.g., only tightly bound polymer at less than 0.8 mg/m2), the plasticizer was effectively excluded from the polymer chains at the silica−polymer−air interface. When excluded from the tightly bound polymer, the plasticizer existed in an environment that was similar to that of pure plasticizer. At larger adsorbed amounts, the plasticizer was effective at lowering the Tg of the adsorbed polymer.

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INTRODUCTION

Others have identified a reduction in the Tgs of adsorbed polymers on surfaces without any strong interaction between the polymers and the surfaces.8,9 Clearly, the behavior of the interphase depends on the interaction between the polymer and substrate and can easily be affected by surface treatment10 as well as the size and nature of the particles themselves.11,12 Several techniques have been used to study these Tgs, including thermal analysis,6,7,13 ellipsometry,5,14 dielectric relaxation,15−17 NMR,18−26 and ESR.27,28 Many factors are known to affect the Tg of a polymer,1,29 including the addition of fibers, fillers, and small molecules, specifically plasticizers, into the polymeric material. In one view, plasticizers increase the total free volume of the system, thereby allowing long-range segmental motion to occur at reduced temperatures.29,30 As a result, desired properties can be achieved, such as flexibility, softness, reduced stiffness, shock

A polymer composite is a multicomponent polymeric material such as a polymer blend, plasticized polymer, structured latex, or filled system with adsorbed polymer.1,2 Polymer composites are useful because of their superior performance, as compared to bulk polymers. The structural and functional properties of polymer composites depend upon the properties and interactions of their components, especially at interfaces and in interphases where the different components come in contact. An understanding of the behavior of polymers at interfaces is essential and beneficial for improving their properties or for developing novel materials for specific uses. Many aspects of the behavior of polymers at interfaces have been studied extensively.1,3 The investigations of glass transitions (Tg) and the dynamics of adsorbed polymers are particularly relevant to the current work. It has been shown that the Tgs of adsorbed polymers can be quite different from those of bulk polymers.4 Some studies have found increases in the Tgs of polymers adsorbed on attractive surfaces with specific interactions, such as with hydrogen or covalent bonding.5−7 © XXXX American Chemical Society

Received: January 4, 2017 Revised: February 14, 2017

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DOI: 10.1021/acs.macromol.7b00022 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules resistance, ductility, and processability.31−33 Plasticizers can also be used in conjunction with fillers or fibers, lowering the Tgs of the polymer matrix.34−36 Rigid fillers and fibers generally tend to increase Tgs, at least locally. The combined effects of fillers and plasticizers on polymers can produce many interesting properties, such as enhanced conductivity and rigidity in polymer electrolytes.34,37 The addition of plasticizer can also facilitate the dispersion of fillers and improve the fluidity of composites.35 On the other hand, increasing the amount of filler usually reinforces a material with greater storage modulus and a higher Tg.35 Moreover, filler is sometimes used to reduce the loss of plasticizer in films36 or composites38,39 with the result depending on the nature of the filler, polymer, and plasticizer. In any case, it is useful to understand polymer− filler−plasticizer systems. We have been interested in the behavior of adsorbed polymers, particularly the characterization of tightly bound polymers.7,40−43 Some of the prior work has focused on the behavior of poly(vinyl acetate) (PVAc) on silica using calorimetry,40 deuterium NMR,18 FTIR,40,44 atomistic simulation,45,46 and dielectric relaxation.17 For adsorbed PVAc on silica, a clear picture has emerged. At very small adsorbed amounts, on the order of 0.8 mg/m2, the adsorbed polymer consists of tightly bound polymer with a higher temperature/ broader glass transition and mobility less than the bulk polymer. This behavior was dominated by the H-bonding of the polymer to the silica. At larger adsorbed amounts, an intermediate, bulklike polymer, with a glass transition slightly higher than bulk, formed. Eventually, at the air interface, there existed a more-mobile polymer with lower temperature glass transition and higher mobility than the bulk polymer. The tightly bound, intermediate (bulklike), and more-mobile polymer segments could all exist in a layer of about 2 nm thickness or less. Di(propylene glycol) dibenzoate (DPGDB) is an effective plasticizer for PVAc, and its effects on lowering the glass transition of PVAc and the dynamics of the polymer have been studied with calorimetry and 2H NMR.24,47 For the adsorbed polymer on silica, there was no effect of the plasticizer when the adsorbed amount of polymer was 0.81 mg/m2.47 This adsorbed amount was approximately the amount of tightly bound PVAc on silica.45 However, at larger adsorbed amounts of 1.42 or 1.81 mg/m2, the plasticizer was effective at lowering the Tg of the adsorbed polymer.25 In the present work, we answer the question of why the plasticizer was ineffective at lowering the Tg of the tightly bound polymer. We do this with 2H NMR and probe the dynamics of deuterated plasticizer, DPGDB in contact with poly(vinyl acetate) (PVAc) on silica, via the motions of the phenyl ring. The ineffectiveness of DPGDB could be rationalized based on two possible mechanisms as schematically depicted in Figure 1. One possibility is that the plasticizer molecules were immobilized in a rigid polymer−silica interfacial layer that did not allow the plasticizer to move. Another possibility is the exclusion of the plasticizer from the polymer at the interphase (i.e., the tightly bound polymer). Our use of 2H NMR with a deuterated plasticizer,47 and our previous studies on PVAc-d3,24,25 allowed us to distinguish between these two possibilities and answer this question authoritatively.

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Figure 1. Two plausible mechanisms for the ineffectiveness of the plasticizer in tightly bound PVAc: (a) plasticizer embedded in an immobilized layer of PVAc and (b) plasticizer excluded from the adsorbed PVAc. reacted with thionyl chloride (Aldrich, Milwaukee, WI) via the nucleophilic substitution reaction to yield benzoyl chloride-d5 (Scheme 1). In a 50 mL two-necked round-bottom flask equipped with a drying

Scheme 1

tube, 5.017 g (0.0395 mol) of benzoic acid-d5 was added, followed by a dropwise addition of 4.070 g (0.0342 mol) of thionyl chloride. The reaction was stirred using mild heat and refluxed for 1−1.5 h, and pH paper was used to check that all of the hydrogen chloride gas was evaporated. A dark yellow or brownish crude product was found and further purified by distillation until colorless (or clear pale yellow). The yield of this purified product was 83%. Synthesis of Deuterated Plasticizer, Di(propylene glycol) Dibenzoate (DPGDB-d10).33 The deuterated plasticizer was prepared using an esterification reaction of 4.141 g (0.0285 mol) of prepared benzoyl chloride-d5 and 1.925 g (0.0143 mol) of di(propylene glycol) (Aldrich, Milwaukee, WI) in a 50 mL two-necked round-bottom flask equipped with a drying tube. The reaction (Scheme 2) was refluxed at 90−110 °C for 2.5 h or until the hydrogen chloride gas was gone. After the reaction mixture cooled to room temperature, 20−25 mL of diethyl ether (Aldrich, Milwaukee, WI) was added. The mixture was separated using a separatory funnel and washed with 20 mL of water; the aqueous layer was discarded. A 20% w/w Na2CO3 (Aldrich) solution was used to wash the resulting organic layer until it became neutral (checked with pH paper). After washing, the Na2CO3 solution layer was discarded. The organic layer of the diethyl ether was evaporated, resulting in a brownish viscous liquid. The crude product was further purified by dissolving it again in diethyl ether and then adding decolorizing activated charcoal (neutral) (Aldrich). The solution mixture, with activated charcoal, was stirred for 15 min. The liquid portion was then decanted, and the solvent, diethyl ether, was evaporated. The finished product was a clear pale yellow viscous liquid. 1H NMR (400 MHz, CDCl3) δ: 5.30 (m, 1H), 4.27 (m, 1H), 3.66 (m, 4H), 1.34 (m, 6H). There was no visible intensity of the aromatic protons, suggesting a high degree of deuterium incorporation. Preparation of Plasticized-Adsorbed Polymers. Solutions of known amounts of PVAc (Mw = 170 kDa, Scientific Polymer Products, Inc., Ontario, NY), and plasticizer (as described above) were prepared in test tubes using toluene as the solvent (Aldrich, Milwaukee, WI). These solutions were shaken in a mechanical shaker for 12 h, and then 0.3 g of silica (Cab-O-Sil M5, 200 m2/g) was added. These mixtures were shaken by a mechanical shaker for 72 h at room temperature. The toluene was removed by passing air (using a glass pipet with a slow flow rate) through the adsorbed polymer−silica mixtures while they were agitated. The drying of the absorbed samples was completed in a vacuum oven under 30 mmHg at 60 °C for 72 h. Two adsorbed samples were prepared at 2.2 and 0.8 mg/m2. These samples will be

EXPERIMENTAL SECTION

Synthesis of Deuterated Benzoyl Chloride-d5.47,48 Benzoic acid-d5 (Cambridge Isotope Laboratories, Inc., Andover, MA) was B


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Figure 2. Thermogravimetric analysis (TGA) plot of 37% plasticized-adsorbed PVAc on silica: (A) larger adsorbed-amount sample with 2.60 mg/m2 and (B) smaller adsorbed amount sample with 0.76 mg/m2 PVAc on silica. Characterization of the Plasticized-Adsorbed Polymers by Solid State Deuterium (2H) NMR. The 2H NMR spectra were obtained using a Tecmag Discovery 400 MHz NMR spectrometer equipped with a high-power amplifier, a fast digitizer, and an Oxford AS-400 wide bore magnet. A fixed-frequency wide-line probe (Doty Scientific, Columbia, SC) with an 8 mm (diameter) sample coil was used. For surface samples, the 8 mm coil size provides good sample volume for these low concentration samples. The quadrupole-echo pulse sequence (delay−90y−τ−90x−τ−acquisition) was used with a 2 H frequency at 61.48 MHz. The 90° pulse width was 2.8 μs, and an echo time (τ) of 30 μs was used. For each temperature, the probe response was tuned before collecting the spectrum. The raw data were left shifted so that the Fourier transform was started from the top of the echo. Approximately 20000−110000 scans were collected for adsorbed samples, depending upon the operational temperature, and no line broadening was used. The spectra were taken from a range of −20 to 40 °C at intervals of 5 °C, depending on the Tg of the samples. Temperature was controlled with an accuracy of ±1 °C. Using the Mestrec software package (Santiago de Compostela University, Spain), the spectra were processed and scaled to the same height for comparison. NMR Simulations. The experimental NMR line shapes were simulated using a FORTRAN program known as MXQET.49,50 The information about the motional rates and types of motion was obtained from these simulations. The NMR line shapes were sensitive to changes in the motion that ranged between 104 and 107 s−1. The 2H NMR line shapes of aromatic ring motions in a solid can often be the results of a rigid lattice (no motion) with a splitting between the two horns of about 120−135 kHz, 180° jumps (D/4 = 30−34 kHz), and continuous diffusion about their symmetry axes (D/8 = 15−17 kHz).51 In this work, the simulations were based on the phenyl ring motion modeled as a two-site jump, 180° ring flips, with an asymmetry

referred to as those with larger and smaller adsorbed amounts (i.e., the plasticizer was not included in the adsorbed amount reported). The amount of adsorption was controlled because the only the solvent was evaporated in the drying process, leaving the polymer, plasticizer, and silica behind. The adsorbed amounts were verified with thermogravimetric analysis (TGA, vide infra) and consistent with that determined by the compositions of the starting solutions, amount, and specific surface area of the silica. The amount of plasticizer in these adsorbed samples was 37% w/w of plasticizer/polymer. Characterization of the Plasticized-Adsorbed Polymers by TGA and TMDSC. High-resolution thermogravimetric analysis (HiRes TGA 2950) (TA Instruments, New Castle, DE) was used to quantify the polymer and the plasticizer content. The scans were run at a heating rate of 20 °C/min in the high-resolution sensitivity mode, beginning at room temperature and progressing to 750 °C in air. A temperature-modulated differential scanning calorimetry instrument (MDSC 2920) (TA Instruments, New Castle, DE) was used to measure the Tg. The sample pans were referenced against empty pans. Two heating scans and one cooling scan were made using the following procedure: a constant (isothermal) temperature was maintained at −40 °C for 5 min and then raised to 150 °C at a rate of 2.5 °C/min with a modulation amplitude of ±0.5 °C for a period of 60 s, isothermal for 3 min, cooled to −40 °C at the same rate, and then isothermal for 3 min. A mass of approximately 7−10 mg of samples was used, and the cell was purged with nitrogen gas at 50 mL/min during the scans. After the first heating and cooling scan, the second heating scan was run under the same conditions as the first heating scan. The Tg was determined based on this second heating scan. Results were shown as plots of differential reversing heat flow rate (dQrev/dT) vs temperature. The peak of the derivative of the reversing heat flow rate versus temperature was assigned as the Tg. C


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Figure 3. 2H NMR spectra of deuterated plasticizer di(propylene glycol) dibenzoate-d10 (DPGDB-d10).

Figure 4. Experimental (black) and simulated (gray) 2H NMR spectra for 37% plasticizer (DPGDB-d10) with PVAc for the larger adsorbed amount (2.60 mg/m2) sample. experimental line shapes were then fitted with a superposition of the simulated spectra by using the MATLAB mathematical program (The Mathworks, Inc., Natick, MA). The weight fractions of each of the simulated spectra were found by minimizing the differences between the experimental spectra and the sum of the simulated ones. A constrained least-squares fit was applied to find the absolute weight fractions of a series of spectra.

parameter, η = 0.04. Several authors have reported an asymmetry parameter for phenyl ring in the range of 0.03−0.06.49,52−55,42 With a fast exchange rate, k > 108, this 180° jump about the C2 symmetric axis do not result in isotropic motion.47,50 In order to obtain an isotropic spectrum, the addition of isotropic axial of motion was required to further average the quadrupole coupling constant (qcc). For this work, since the isotropic component was not our focus, it was sufficient to add a Gaussian single central component to represent it. The D


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The 2H NMR spectra for the 37% plasticized DPBDB-d10 in an adsorbed PVAc sample with smaller adsorbed amount of 0.76 mg/m2 as a function of temperature are shown in Figure 5.

RESULTS Since the signal intensities of NMR spectra depend on the concentrations (or amounts) of the samples within the coils of the probe,56 and only a limited amount of adsorbed sample can be packed into a NMR tube that will fit into a coil, the amount of plasticizer in an adsorbed sample was small. Accordingly, a high ratio of plasticizer to polymer was used to overcome the problems resulting from weak signal intensities. In this work, the adsorbed polymers contained 37% w/w (plasticizer/PVAc). This approximate amount is commonly found in soft plastic toys and other devices, but the plasticizer content can vary between 15 and 60%, depending upon the application.57 Two samples with different adsorbed amounts were prepared and used throughout this study. The smaller adsorbed amount (0.76 mg/m2) was approximately the amount of tightly bound PVAc on silica.45 The larger amount (2.60 mg/m2) was well above that amount. Thermogravimetric analysis (TGA) was used to analyze the amount of plasticizer and polymer in each sample. Figure 2 shows the decomposition thermograms of adsorbed samples containing 37% w/w plasticizer/PVAc for both the larger adsorbed amount (2.60 mg/m2) and 0.76 mg/m2 for the smaller adsorbed amount sample. The plasticizer decomposed in the range of 290−325 °C for the larger adsorbed amounts and at approximately 270−325 °C for the smaller adsorbed amounts. The polymer started to decompose at a higher temperature of about 350 °C.24,47 The amount of plasticizer estimated from the TGA experiments was within 5% of those estimated from the compositions of the original mixtures. The 2H NMR spectra of the pure plasticizer (DPGDB-d10) were taken at various temperatures ranging from −60 to +24 °C. Each spectrum at a different temperature was scaled to the same height for comparison. Low intensities of the 180° ring flip (the two inner horns) were observed in the low temperature range, as shown in Figure 3. The Tg (NMR) of the plasticizer, DPGDB-d10, was roughly between −30 and −20 °C and taken to be approximately −25 °C. At −20 °C, collapse of the Pake powder pattern was almost complete, replaced by a relatively narrow component at the center. As the temperature increased, the central component gradually became a sharp resonance at 24 °C, as seen in Figure 3. The quadrupole-echo 2H NMR spectra of a 37% plasticizedadsorbed sample (deuterated plasticizer) with an adsorbed amount of 2.60 mg/m2 were taken as a function of temperature and are shown in Figure 4. The spectra showed splittings of about 125 kHz between the two outer horns at lower temperatures, indicative of rigid material. The splittings between the two inner horns of about 29 kHz was attributed to the phenyl rings executing 180° ring flips. At low temperatures, from −20 to 0 °C, the spectra showed powder patterns for both rigid material and that with 180° phenyl ring flips. As the temperature increased, however, a narrow central component rose and the Pake powder patterns eventually collapsed. The plasticizer Tg of a 37% plasticized-adsorbed sample at 2.60 mg/m2 was assigned as 24 °C. The determinations of the plasticizer Tgs were consistent with those reported in previous work25,47 that defined Tg for heterogeneous line shapes as the temperature at which the signal height of the narrow resonance of a spectrum is approximately three times that of the height of the horns. The estimation uncertainties of these plasticizer Tg assignments less than ±5 °C.

Figure 5. Experimental (black) and simulated (gray) 2H NMR spectra for 37% plasticizer (DPGDB-d10) with PVAc for the smaller adsorbedamount sample (0.76 mg/m2).

The NMR glass transition took place roughly between −16 and 0 °C and was centered at plasticizer Tg (NMR) of about 10 °C. As temperature increased, the narrow resonance of the central component also increased. As shown in Figure 5, at 0 °C, only a slight residue of the Pake powder pattern remained and that became almost negligible at a higher temperature of about 24 °C. A summary of the plasticizer Tgs for the pure plasticizer, 37% plasticized-bulk PVAc, and 37% plasticized-adsorbed PVAc (including larger and smaller adsorbed amount samples) as determined by NMR is shown in Table 1. Comparisons of the 2H NMR spectra of the pure plasticizer, 37% plasticized bulk polymer,47 and 37% plasticized-adsorbed samples were made at a temperature of about 24 °C, as shown in Figure 6. While the spectrum of a sample containing a larger adsorbed amount, 2.60 mg/m2, still showed a significant amount of the Pake powder pattern, the rest of the 2H NMR spectra showed only the narrow resonance of the central E


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site jump model of a phenyl deuteron executing 180° ring flips was used for the simulations. Fitting the simulations and experimental spectra was done using a MATLAB program. A broad distribution of spectra with different jump rates was required for a good fit. The distributions of (log) jump rates as a function of temperature and plasticizer content are presented using bar graphs in the Supporting Information. The fitted results represent the regimes of major motional components as slow, intermediate, and fast with respect to their jump rates. It is known that even with the fast exchange rates, k > 108 Hz, the 180° jumps about the C2 symmetry axis did not result in isotropic motion. The isotropic motion, resulting in a narrow resonance, occurred at fast exchange rates when the spectrum was no longer sensitive to the jump rate. In the slow regime (k < 104 Hz), the phenyl ring flip motion resulted in Pake powder patterns with rigid components in which there were no significant loss of intensity during the quadrupole echo. A loss in spectral intensity was observed as temperatures approached the glass transition. For the simulations, a significant intensity loss was found58 as the exchange rate became faster, about 104−107 Hz (classified as the intermediate regime). This regime corresponds to the glass transition of a material where the collapse of a Pake pattern with rigid components and the rise of narrow resonance spectra of a liquid component are observed. Again, for the fast regime, k > 108 Hz, there was little intensity loss of echo since liquidlike components are mostly found in this regime. From the NMR fittings of the spectra, we were able to calculate the distributions of the jump rates as either the simple average or the average of the logarithms of the rates. Plots of the log⟨k⟩ and ⟨log k⟩ are shown in the Supporting Information for the 2.60 mg/m2 sample. Unfortunately, the plots for the adsorbed samples were difficult to interpret as there was a great deal of heterogeneity of the samples as evident in the spectra. A comparison with the more homogeneous bulk samples seems unwarranted. Temperature-modulated differential scanning calorimetry (TMDSC) was used to measure the glass transitions of the adsorbed 37% plasticizer samples for the larger and the smaller adsorbed amounts. The TMDSC thermograms of plasticizedadsorbed PVAc samples are shown in Figure 7. The derivative of the reversible heat flow rate was plotted as a function of temperature for the different adsorbed amount samples. The maximum in the derivative reversible heat flow rate curve was assigned as the Tg (TMDSC). For the 37% plasticized-bulk PVAc sample, only one transition was found with a Tg (TMDSC) of about −7 °C and a transition width of about 25 °C. On the other hand, the adsorbed sample with the larger adsorbed amount, 2.60 mg/m2, showed two transitions: the loosely bound polymer (bulklike) and tightly bound polymer.7,25Although the two transitions found here were not as distinct as some in other studies which used a more sensitive calorimeter,7,45,59 the second transition appeared more like a shoulder of the first one. The Tg (TMDSC) observed for the loosely bound transition of 37% plasticized-adsorbed 2.60 mg/ m2 PVAc was about 15 °C, with a transition width of about 30 °C. Nevertheless, no clear transitions were observed for the sample with the smaller adsorbed amount, 0.76 mg/m2. This is likely due to the broadened transition due to the plasticizer plus the small amount of polymer in the sample. In nonplasticized adsorbed PVAc, Mortazavian et al.45 showed that the tightly bound PVAc had a significantly elevated and broadened glass transition relative to the bulk polymer. The

Table 1. NMR and TMDSC Glass Transition Temperature Rangesa,b for Bulk Plasticizer and 37% Plasticized Samples NMR (from DPGDBd10) sample bulk plasticizer plasticized PVAc plasticized PVAc on silica, adsorbed amount 2.60 mg/m2 plasticized PVAc on silica, adsorbed amount 0.76 mg/m2

TMDSC

Tonset

Tmid

width

TA

width

−30 5 0

−25 15 24

5 10 24

−53c −7 15

n/a 25 30 (TA)

1 mg/m2), a second peak, slightly above the Tg of bulk polymer, 44.2 ± 0.3 °C, was found and corresponded to loosely bound polymer. For adsorbed PVAc on silica, the amount of tightly bound polymer has been reported to be 0.78 mg/m2.45,59 Therefore, the larger adsorbed amount sample contained more than the tightly bound amount for the nonplasticized system and the smaller adsorbed amount sample had a composition close to that tightly bound amount.

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DISCUSSION Solid-state NMR has been shown to be a powerful technique used to probe the molecular motion in polymeric systems,22,51,60−63 in particular, in adsorbed,19,20 composite,64 and plasticized/diluent65−69 polymer systems. In many cases, analysis of NMR line shapes and relaxation times can provide specific information about the rates and types of motion. Our research group has focused on using 2H NMR to study the dynamics of polymer in bulk and in adsorbed samples and shown that the heterogeneity of surface species was particularly apparent with this technique.18,20,21,23−25,47,70 In our spectra of deuterated plasticizer, a Pake powder pattern was observed for a rigid component at low temperatures for the DPGDB-d10. The quadrupolar splitting for a deuterium nucleus, Δνq, is given by58,71,72 Δvq =

3 2 (e qQ /h)(3 cos2 θ(t ) − 1 − η sin 2 θ(t ) cos2 φ(t )) 4 (1)

where e qQ/h is the quadrupole coupling constant (qcc); θ and φ are the Euler angles in the principal-axis system. These angles define the relative orientation of the electric field tensor with respect to the applied magnetic field β0, and η is the asymmetry parameter which defines the shape of the powder pattern. The asymmetry parameter is usually zero for aliphatic C−D bonds, indicating that the electric field gradient tensor is axially symmetric.72,73 However, for aromatic C−D bonds, the 2

G


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than that in the corresponding bulk polymer (Figure 8 in our previous work).47 This suggested to us that the plasticizer was less effective and more restricted than in the bulk polymer. This was expected based on the fact that the loosely bound polymer was still somewhat restricted on the surface,18,45 and the plasticizer definitely penetrated the loosely bound polymer. In contrast to the behavior of the 2.60 mg/m2 sample, the 0.76 mg/m2 adsorbed polymer sample behaved very differently. We found an unexpectedly large decrease in the plasticizer onset of motion compared to the bulk system with the same amount of plasticizer. While this onset of motion temperature was higher than that for the pure plasticizer, possibly because a slight interaction with the tightly bound polymer, it seems to represent plasticizer molecules little constrained by polymer. The DPGDB-d10 spectra were very similar in bulk and in the system with the tightly bound polymer (Figure 6). We also note that while there is a very large increase in the plasticizer mobility, it had no effect on the polymer Tg as previously reported.25 Clearly, the plasticizer is not immobilized in the tightly bound polymer. Therefore, we conclude that the plasticizer was largely excluded from the tightly bound polymer. Similar observations have been found in other work using neutron scattering and echo techniques. Shimomura et al.43 have identified that a tightly bound layer (about 5 nm) found in cross-linked polyisoprene on silica showed little or no penetration of deuterated n-hexane, a good solvent for polyisoprene. In the case of polybutadiene on carbon black, Jiang et al.42 found that a 0.5 nm layer of “inner” polymer was found to be near the particle surface and unswollen with deuterated toluene. These studies provide results similar to ours in terms of penetration of diluent into the tightly bound layer. However, our study includes a sample with only tightly bound polymer. Interesting, the distance scales for tightly bound polymer in both of these systems are similar to that in adsorbed PVAc. The role of mutual interactions and layering in polymer− nanoparticle systems is important and complex.90 In this case similar interactions between the polymer segments and plasticizer occur for both tightly and loosely bound PVAc, one might wonder why the plasticizer penetrates the loosely and not the tightly bound segments. The difference is fundamentally due to the significant attractive interaction between the PVAc and the silica surface. FTIR and simulation studies confirm the attachment of polymer carbonyls to surface silanols via hydrogen bonding.17,44−46 Based on FTIR results, hydrogen bonding appears to involve virtually all of the surface silanols in the silica/PMMA system40 and also for PVAc44 at adsorbed amounts similar to the tightly bound amount. The PVAc/silica spectra are shown in the Supporting Information. Therefore, it is reasonable to assume that polymer effectively covers the surface of the particles. These H-bonding attachments are responsible for an increased density of polymer near the silica interface,45,46 resulting in less free volume and consequently a reduction in mobility of the tightly bound layer.17,18 Plasticizer does not seem to penetrate the denser, less mobile, more highly constrained material.

sample being much faster than that in the larger adsorbed amount sample. Second, there is an unexpectedly large reduction in the temperature for the onset of high mobility for the plasticizer in the adsorbed sample with the small adsorbed amount. Third, there was great similarity in the narrow resonance spectra of plasticizer in the smaller adsorbed amount sample and that of the pure plasticizer. The dynamics of the plasticizer in bulk PVAc samples were previously reported.47 Here, as shown in Figure 3, the dynamics of the plasticizer in the adsorbed sample with the larger adsorbed amount moved slower than in bulk plasticizer. Although, the rise in the central narrow resonance seemed to occur at about the same temperature (about 0−5 °C) for both bulk and adsorbed (larger adsorbed amount) samples, the collapse of the powder pattern in the bulk sample occurred at lower temperature. Several studies have also observed that the dynamics of bulk samples moved faster than those of adsorbed samples.25,79−84 On the other hand, as shown in Figure 5, the dynamics of DPGDB-d10 in the sample with the smaller adsorbed amount seems to be much faster than that found in the bulk plasticized PVAc samples or the sample with the larger adsorbed amount. The collapse of the powder pattern was seen at a lower temperature (−10 °C), and only a small residual powder pattern was found at 0 °C. Nevertheless, the dynamics of plasticizer in the smaller adsorbed amount sample seemed to be somewhere between those of bulk and those of pure plasticizer, as seen in Figure 3, but clearly much more like the bulk plasticizer. It was not at all like that of the bulk-plasticized system. It is known that the Tgs are good indicators and reflect the plasticization process. Hence, the effect of plasticizer can easily be observed through the Tgs. The reduction in Tg of thin polymeric films such as PVAc, poly(styrene), and poly(methyl methacrylate) due to the presence of diluents such as water or plasticizers has been reported.9,67,85−87 The effect of plasticizer on Tg reduction has also been found in supported PMMA films on silica.88 However, many of these studies have focused on films of thicknesses much greater than than the amounts reported herelittle or no work on such small adsorbed amounts around a nanometer or so. This does not imply that there are no other longer range interactions that are important in composites.89 Several studies have compared the Tgs (NMR) of bulk and adsorbed polymers in the absence18,19,23−25 and including the presence of plasticizer.9,24,25,47 Nambiar and Blum have reported a higher Tg of polymer for silica-adsorbed PVAc-d3 than that of bulk PVAc-d3 in both the absence and presence of plasticizer.24,25 In addition, they also observed that the weighted average of adsorbed PVAc-d3 Tgs decreased with increased adsorbed amounts as determined by both NMR and TMDSC experiments.25 In contrast to the behavior monitored by the 2H NMR spectra of adsorbed PVAc-d3,25 the collapse of the powder pattern for DPGDB-d10 in the smaller adsorbedamount sample occurred at a temperature that was lower than that in the bulk system. In our previous work,47 for this polymer−plasticizer system, the onsets of molecular motion for the plasticizer and polymer were found to be similar given by the Tg (NMR). The plasticizer Tg of bulk PVAc containing 37% w/w plasticizer was reduced by 56 °C while the sample with larger adsorbed amount had its plasticizer Tg reduced by 47 °C compared with that of the pure polymer. For the surface system with 2.60 mg/ m2, the onset of mobility was increased several degrees higher

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CONCLUSIONS H NMR has effectively probed the dynamics of the plasticizer, DPGDB-d10, in conjunction with adsorbed PVAc on silica. The dynamics of plasticizer was in the adsorbed samples was found to be more heterogeneous than that observed in bulk samples, and the spectra successfully fitted with models containing rigid 2

H


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Macromolecules

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jumps and isotropic components. The DPGDB-d10 plasticizer had little or no effect on the tightly bound adsorbed PVAc on silica. The 2H NMR findings provided solid evidence that of the effect of plasticizer in adsorbed polymers differs as a function of adsorbed amounts. At larger adsorbed amounts, the plasticizer was moderately effective, but an unexpectedly large decrease in temperature of the onset of the motion of DPGDB-d10 occurred in the tightly bound amount samples. The narrow resonance spectra of samples with smaller adsorbed amounts of polymer were similar to those of the pure plasticizer. This evidence that we obtained from 2H NMR allowed us to distinguish between two reasons for why the plasticizer did not lower the Tg of the tightly bound adsorbed polymer, namely, (i) it was trapped rigidly in the tightly bound polymer or (ii) it was excluded from the tightly bound polymer. We conclude that with a small amount of adsorbed polymer (i.e., tightly bound polymer) the DPGDB-d10 was excluded from penetrating the adsorbed PVAc at the polymer−air interface. Hence, it had very little or no effect on the dynamics of the tightly bound polymer.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00022. Figures S1−S4 and Table S1 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(F.D.B.) E-mail: [email protected]. ORCID

Frank D. Blum: 0000-0002-7884-3134 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the National Science Foundation under Grant DMR-1005606, the Missouri University of Science and Technology, and Oklahoma State University. The authors gratefully thank Raymond Kendrick for his technical support, Dr. Robert O’Connor for useful discussions, Dr. Cyriac Kandoth and Dr. Waraporn Viyanon for their help on Fortran programming, and Dr. Madhubhashini Maddumaarachchi for supplying the FTIR spectra.

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