Film Formation of Pressure-Sensitive Adhesives - ACS Publications

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Film Formation of Pressure-Sensitive Adhesives (PSAs) Studied with Förster Resonance Energy Transfer (FRET) and Scattering Intensity Hares Wahdat,† Christopher Hirth,† Diethelm Johannsmann,† Matthias Gerst,‡ Markus Rückel,‡ and Jörg Adams*,† †

Institute of Physical Chemistry, Clausthal University of Technology, D-38678 Clausthal-Zellerfeld, Germany Advanced Materials & Systems Research, BASF SE, D-67056 Ludwigshafen, Germany

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‡

ABSTRACT: The drying of industrially relevant latex dispersions designed for use as pressure-sensitive adhesives (PSAs) was followed using Förster resonance energy transfer (FRET) and scattering intensity as indicators for the progress of film formation. FRET and scattering intensity report the state of polymer interdiffusion and of particle deformation, respectively. Because the exciting UV-radiation only penetrated a few micrometers deep into the film, FRET measurements undertaken from the top and the bottom yielded different results. The combination of the two evidenced skin formation. Particle deformation occurs in two steps. There is a significant, but incomplete, decrease in turbidity because of skin formation. Only after the drying front has propagated to the substrate, the top layer turns fully clear. This second step is interpreted as coalescence, meaning the breakup of lamellae separating particles. Coalescence is followed by a sharp increase in interdiffusion. Further aspects studied included crosslinking, hydroplasticization, and tackifying resins.

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INTRODUCTION Pressure-sensitive adhesives (PSAs) are polymeric materials which reversibly adhere to surfaces upon application of light mechanical pressure.1,2 PSAs find use as for instance, labels, adhesive tapes, and temporary protective films. In terms of materials employed, they are often based on natural rubber, styrenic block copolymers, or acrylics. They may be prepared from solutions (natural rubber and acrylics), from hot melts (styrenic block copolymers), or from water-borne dispersions (acrylics).1 The polymer’s viscoelasticity governs tack, peel strength, and shear-holding power.3 Acrylic PSAs have a low glass transition temperature, Tg, and thus are tacky as such. They lack cohesive strength, though,3 and therefore need to be cross-linked covalently or noncovalently.4 Acrylate-based PSAs are often copolymerized with acrylic acid or methacrylic acid to increase adhesion toward polar surfaces. This study is concerned with acrylic PSAs dried from latex dispersions. Latex films are environmentally friendly in that they release only small amounts of volatile organic compounds (VOCs) into the atmosphere while they dry. Unfortunately, the performance of PSAs prepared from latexes is slightly inferior to the performance of solution-dried PSAs, which can be attributed to structural heterogeneity.5 Drying water-borne PSAs go through the “film formation process”.6 Film formation encompasses three stages,7 commonly described as the evaporation of water (stage I), particle deformation (stage II), and polymer interdiffusion (stage III).8 The two most important system parameters governing film formation are the speed of drying and the softness of the polymer phase.9 PSA formulations differ from typical coatings formulations in that © XXXX American Chemical Society

the soft spheres used in PSAs readily deform during stage II, while particle deformation may be a problem for coatings. For a related reason, PSA formulations more often develop a skin, that is, a layer of deformed particles close to the film−air interface, which impedes the further transport of water and thereby slows down drying.10 Whether or not a drying film is prone to skin formation also depends on the speed of evaporation, where fast drying promotes skin formation. Polymer interdiffusion (stage III of the film formation process as described by Vanderhoff) is crucial for mechanical robustness.11 Again, PSA formulations and coatings formulations behave much differently. Because of the low Tg, interdiffusion is very fast at room temperature. In coatings formulations, the high Tg reduces the mobility of the polymer chains and thereby delays interdiffusion.12 At room temperature, polymer interdiffusion in coatings formulations can be slow to the extent that it limits the final film’s cohesion, and therefore it has been studied in some depth. Early investigations were based on neutron scattering.13,14 An instrumentally less demanding approach consists in fluorescent labeling of polymer chains and inferring the degree of interdiffusion from the efficiency of Fö rster resonance energy transfer (FRET) between different dye molecules.15 Recently, the formation of excimers from pyrene has been demonstrated as an alternative route of access to polymer interdiffusion.16 The work reported here relies on FRET as the indicator of interdiffusion. Received: February 26, 2018 Revised: June 6, 2018

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DOI: 10.1021/acs.macromol.8b00423 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules The early FRET studies on interdiffusion targeted high-Tg materials designed for use as coatings. These required thermal annealing to fully interdiffuse. Annealing leaves ample time for intermediate quenching and the study of FRET.17,18 Spatial heterogeneity on the macroscalewhich is present and is of much importance in film formationwas largely absent. These and the following studies (also including low-Tg polymers)19 aimed at the properties of the respective materials, which later included cross-linked polymers,20−22 and polymers plasticized by water.23−25 The film formation process itself came into focus later.26,27 This development was partly driven by the advent of pulsed UV-LEDs with high repetition rate (2 MHz in the studies reported below) at moderate cost. FRET studies on drying films give much insight into the details of the film formation process. There is a complication, though, because the spot under study may or may not be representative of the entire film. Given that some films dry from the edge to the center (“edge-in-drying”, to be distinguished from “top-downdrying”), the kinetics of interdiffusion at some location of the film depends on the time at which a lateral drying front (if it exists) passes this location. It is helpful to monitor the light scattered from the spot under study in parallel to the FRET signal.28 Scattering monitors particle deformation, while FRET reports interdiffusion. Monitoring bothat the exact same spotallows to correlate particle deformation with interdiffusion. A first motivation of this work was to elucidate in which ways the drying of PSAs differs from the drying of coatings. The most important of these differences is that PSAs are more prone to develop a skin than coatings.10 As will be shown, skin formation is readily discerned in the combined FRET/ scattering data. In particular, the FRET/scattering kinetics differ between experiments where the sample is viewed from above and from below. In a second step, we expand and show how the FRET/scattering combination can give access to further details of the material’s film formation characteristics. This concerns the consequences of partial cross-linking, hydroplasticization, and the influence of tackifiers. We briefly comment on these effects in the following. • Many PSAs are lightly cross-linked. Cross-links provide cohesion to strengthen and sustain fibrils so that they do not flow and break.1 If the latexes are partially crosslinked before film formation, cross-linking will limit interdiffusion. For this reason, it is attractive to delay cross-linking after film formation is complete. • Hydroplasticization is known to aid film formation in both the particle deformation stage and the interdiffusion stage.24,25 Hydroplasticization has the consequence that slow drying promotes interdiffusion. • PSAs often contain tackifying resins (“tackifiers”). Tackifiers make the material more rigid on short time scales, while they soften the material on longer time scales. In this way, the material gains strength during debonding (a relatively fast process) and at the same time flows more easily during attachment (a relatively slow process), thereby allowing for intimate contact with rough surfaces. Tackifiers consist of small molecules (often polar), which increase Tg. They are antiplasticizers, shifting the rheological spectrum (G(ω) with G the shear modulus) upward and to the left at high frequencies. At the same time, tackifiers dilute the entanglement network, thereby speeding up flow and

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diffusion on long time scales.29,30 They shift the rheological spectrum downward and to the right at low frequencies.

MATERIALS AND EXPERIMENTS

Materials. Latexes were prepared via seeded emulsion polymerization under starve-fed conditions. The polymers in the dispersions generally are copolymers of 2-ethylhexyl acrylate, styrene, n-butyl acrylate, methyl acrylate, and methacrylic acid in a weight ratio of 59/ 20/15/5/1. The polymerization temperature was 80 °C, the overall reaction time was 4 h, and the reaction was carried out under a nitrogen atmosphere. All chemicals were received directly at BASF SE if not mentioned otherwise. All ingredients, except for the monomers, were added as aqueous solutions, and their concentrations are given in wt % with respect to purified water. All chemicals were used as received. Generally, the synthesis procedure was carried out as follows: 0.09 g of a polystyrene seed (30 nm diameter, BASF SE, 33 wt %) and 0.18 g of sodium peroxodisulfate (initiator, 2.5 wt %) were mixed in 19.5 g of purified water, and the mixture was heated up to 80 °C under stirring. Monomers (total mass: 15 g in the aforementioned ratio), 7.6 g of water, 1.62 g of sodium peroxodisulfate, 0.07 g of Dowfax 2A1 (surfactant, 45 wt %, Dow Chemicals), and 0.47 g of Disponil FES 77 (surfactant, 32 wt %) were added as feed within 3 h. The reaction was stirred for another 30 min, and then 2.55 g of purified water were added, and 1.5 g of tert-butyl hydroperoxide (1.5 wt %) and 2.4 g of acetone bisulfite (1 wt %) were fed into the mixture within 30 min. Afterward, the reaction was allowed to cool to room temperature. Donor labeling (“D”) was achieved by adding 1.6 pphm (“parts per hundred monomer”, weight percent with respect to the monomers) of (9-phenanthryl)methyl methacrylate (Phen-MMA, Enamine) as a comonomer. Acceptor labeling (“A”) was achieved with 1 pphm of 1(4-nitrophenyl)-2-pyrrolidinemethyl] acrylate (NPP-A, Enamine). Dispersions with linear chains (“L”) and covalently cross-linked chains (“X”) were prepared. Covalent cross-linking was achieved with 0.5 pphm 1,4-butanediol acrylate. A reference latex without labels (“Ref”) containing linear chains was prepared as well. The Phen-NPP pair was chosen as the FRET pair because the spectra of emission and absorption overlap well and because NPP does not fluoresce and therefore does not interfere with the determination of the donor-decay curves.31 The properties of the latexes are summarized in Table 1. Tg were between −35 and −30 °C. Molecular weights and polydispersities are provided in Table 2.

Table 1. Overview of Investigated Latexesa latex D-L A-L Ref-L D-X A-X

label PhenMMA NPP-A none PhenMMA NPP-A

solids content [%]

dh [nm]

gel content [%]

linear

28.0

154

0

linear linear covalently crosslinked covalently crosslinked

27.1 28.0 28.4

140 149 134

0 0 84

27.4

164

90

chain topology

Dispersities of the hydrodynamic diameter are ≪1.2, solids contents were determined gravimetrically, and gel content is the fraction of the mass of a polymer insoluble in methyl ethyl ketone and the initial mass of the dried film. Serum pH was 2 in all latexes.

a

Characteristically for acrylate-based PSAs, the molecular weight distributions are broad (Mw/Mn > 10). The molecular weight distributions were determined with gel permeation chromatography (GPC) using a differential refractometer (DRI) and a UV-detector sensitive to the respective label (λ = 290 nm for Phen-MMA and λ = 380 nm for NPP-A). The molecular weight distributions obtained with DRI detection were similar to those obtained with UV detection, B


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Macromolecules Table 2. Molecular Weights of the Polymers Obtained with GPCa DRI

UV

latex

Mw [kg/mol]

Mw/Mn

Mw [kg/mol]

Mw/Mn

D-L A-L Ref-L

433 754 299

14.5 25.8 15.7

453 754

11.1 22.3

a

The UV detection wavelengths for D-L and A-L were 290 and 380 nm, respectively.

proving that the labels were distributed evenly between long and short chains. The tackifying resin was Snowtack FH94G, supplied by Lawter Europe. This resin is suitable for acrylates. It is an aqueous dispersion of a fully hydrogenated rosin ester (softening point = 93 °C, initial solids content = 57.6%, d50 = 0.454 μm, d90 = 0.927 μm). The emulsion of tackifying resin was diluted to a solids content of 27.5%. This step ensures that the overall solids content did not change when adding tackifier to the latex dispersion. Preparatory experiments with other, non-hydrogenated tackifiers revealed that the fluorescence from the double bonds contained in these resins was prohibitively strong. Tetrahydrofuran (THF, 99.9%, inhibitor-free, analytical grade), methyl ethyl ketone (99%), trifluoroacetic acid (99%), NaBr (anhydrous, 99%), and sodium dodecyl sulfate (SDS, 98%) were purchased from Sigma-Aldrich. Water was purified using an Arium 611VF ultrapure water system by Sartorius. Silica gel was purchased from Carl Roth. CaCl2 (anhydrous, 99%) was purchased from Merck. Unless mentioned otherwise, the chemicals were used as received. Characterization. Hydrodynamic diameters were determined with dynamic light scattering (DLS) on diluted dispersions in water at 25 °C. Scattered light was detected at 90°. Measurements were carried out using an ALV/DLS system equipped with a multi-τ digital correlator (ALV Germany). Molecular weight distributions were determined by gel permeation chromatography (GPC) using THF containing 0.1 wt % trifluoroacetic acid as eluent and PLgel Mixed-B columns. Temperature was 35 °C. Both DRI detection (Agilent 1100) and UV detection (Agilent 1100 VWD) were employed. Polystyrene standards (0.6 × 103−6.9 × 106 g/ mol) were used for calibration. The glass transition temperature Tg was determined with differential scanning calorimetry (DSC, Mettler Toledo, 3−10 mg of a dried polymer sample, heating curves from T = −80 °C to T = +100 °C at a rate of 10 °C/min). The temperature at the inflection point was identified with Tg. In order to determine the gel content, 0.5 g of a polymer film, which had been fully dried, was swollen in 50 g of methyl ethyl ketone for 4 days without stirring at room temperature. The gel fraction was separated from the solution with a nylon membrane filter (Sefar Nitex, pore diameter 120 μm). The filter with the collected gel was dried and weighed. Gel content was calculated as 100%·mgel/mfilm with mgel the mass of the insoluble polymer collected by the filter and mfilm the initial mass of the film. All gel contents were averages of two measurements. Time-Resolved Fluorescence and Detection of Scattered Light. A sketch of the chamber is shown in Figure 1. The chamber allowed for simultaneous acquisition of scattered excitation light and fluorescence, the latter being analyzed by time-correlated single photon counting (TCSPC, 500 channels with a channel width of Δt = 0.4 ns, electronic components supplied by EG&G). Inferring the amount of FRET from time-resolved fluorescence avoids problems resulting from calibration and scattering. The substrate carrying the film was positioned under the pulsed UV-LED (Picoquant, λ = 290 nm, repetition rate = 2 MHz, 1.4 ns full width at half-maximum). The UV-light was focused to the center of the film, where the width of the focus was 1 mm. The UV-LED and the liquid light guides (Oriel, type 77556) were aligned such that the foci overlapped. Excitation and detection occurred either from above or from below. Given that the depth of penetration of the UV-light for the investigated dispersions

Figure 1. Sketch of the chamber. The sample was studied from either above or below. was 5.5 μm, at most (see the Appendix), observations from the top and the bottom probed the first few micrometers close to the film−air interface and the film−substrate interface, respectively. Fluorescence light passed a long-pass filter (Schott, λ = 360 nm) before being detected by a photomultiplier tube (Hamamatsu). Pulses were passed to the TCSPC unit, which computes decay profiles I(t′). Scattered photons passed an interference filter (Schott, λ = 290 nm) and were counted by a photomultiplier tube (Hamamatsu). Humidity was controlled either by purging with a stream of dried air (3% rH, “convection” in Figure 7) or by placing vials containing humidity-control agents or pure water into the chamber (96% rH). Humidity-control agents were silica gel (10% rH) and saturated solutions of CaCl2 (33% rH) and NaBr (56% rH). Humidity and temperature (21 ± 1 °C) were measured using a digital hygrothermometer (Rotronic Hygromer A1H). This setup has been carefully optimized with regard to control of humidity and temperature. These steps proved to be essential for drawing conclusions from the comparison between data sets. 1/1 (m/m) mixtures of donor- and acceptor-labeled latexes were thoroughly premixed for 20 s before casting. 3 μL of latex was cast onto an ellipsoidal spot with an area of 16 mm2. The wet and the dry thickness were 175 and 50 μm, respectively. The thickness varied slightly across the film. Conventional microscope slides or fused silica plates were used as substrates. Fused silica was needed, when observation occurred from below, because the microscope slides are intransparent to the exciting UV-radiation. Samples were inserted into the measurement chamber immediately after casting. Time t = 0 corresponds to the time of casting within ±5 s. In the kinetic studies, in which the film formation of a drying dispersion was investigated, the accumulation time per decay was Δt = 10 s for the first 300 s and Δt = 30 s afterward. Scattering intensity was measured in 10 s intervals. For determination of constant fit parameters for FRET analysis, decays were recorded until 105 counts were reached at the decay’s maximum. For studies on blends of latex with the tackifying resin, the mass ratio of polymer and tackifier was 3/1. We also studied intermixing in a wet dispersion. These measurements occurred on 1/1 (m/m) blends of D-L and A-L with and without tackifier inside a sealed quartz cell (Hellma, 0.1 mm path length). All measurements were carried out at least twice (duplicates not shown). Data Analysis. Decays curves I(t′) derived from TCSPC were fitted with the two-state-model17 (eq 1), using the Levenberg− Marquardt algorithm. The background as determined from the count rate before the excitation pulse was subtracted from the raw data prior to fitting. The fit function was convoluted with the lamp profile before comparison with the experiment. The final values of χ2 were χ2 < 3 for the kinetic studies and χ2 < 6 otherwise. The two-state model assumes two distinct populations of donors, which are either surrounded by acceptors and perform FRET at the maximum efficiency (as quantified by the parameter 2γ) or are well separated from all acceptors and do not perform FRET at all. The twostate model ignores intermediate situations. The fraction of FRETC


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Macromolecules active donors is called A2. A2 is the central outcome of the fitting procedure. A2 is called “amount of FRET” and quantifies the progress of polymer interdiffusion. The fit function is

Table 3. Constant Parameters Entering the Fit Function (Eqs 1 and 3) Used To Extract the Amount of FRET from the Decay Curves

⎤ ⎡ ⎛⎛ t ′ ⎞ ⎛ t′ ⎞ t′ ⎞ ⎟⎟ + (1 − A 2 ) exp⎜ − ⎟⎥ I(t ′) = I0⎢A 2 exp⎜⎜⎜ − ⎟ − 2γ ⎢⎣ τ0 ⎠ τ0 ⎠⎥⎦ ⎝ ⎝⎝ τ0 ⎠ (1) t′ (in units of ns) is the time elapsed since the UV pulse, I0 is the count rate at t′ = 0, and τ0 is the lifetime of the donor. In principle, τ0 may change as the film dries,32 but for the system studied here, τ0 was 41 ± 0.2 ns (as determined on drying dispersion containing donor only). τ0 was fixed to 41 ns during fitting. The efficiency of energy transfer in a fully intermixed film is quantified by the parameter 2γ, defined by32 2γ =

4 3/2 π NARF3cA 3

fixed parameter τ0 [ns] τ1 [ns] 2γ

⎤ ⎛ t ′ ⎞⎥ + A TR exp⎜ − ⎟ ⎝ τTR ⎠⎥⎦

(2)

Iscat,bsn =

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41 6 1.44

1.93

Iscat,raw(t ) − Iscat,raw(t = ∞) Iscat,raw(t = 10s) − Iscat,raw(t = ∞)

(5)

Figure 2. A typical data set for a blend of donor- and acceptor-labeled dispersions with linear chains dried in a stream of dry air. The purple curve in panel b (open circles) shows the scattering kinetics obtained from a sample with no labels. In this sample, phase 1 is missing because the excitation light is not attenuated by absorption.

turning to the kinetics, we discuss the unusually large starting value of A2,ini. A2,ini is larger than 0.5 for all samples studied here. A more common value is A2,ini ≈ 0.2.17 A first contribution to the high value of A2,ini are fluorescent watersoluble oligomers, which exchange between the particles after mixing donor- and acceptor-labeled latexes. Water-soluble oligomers are a byproduct of emulsion polymerization.33,34 Even wet mixtures of labeled dispersions show an A2 value of around 0.6. DLS measurements on these diluted mixtures gave no indication of particle aggregation or clustering. A second (ubiquitous) source of A2,ini being nonzero even in the absence of acceptor is superpositions of exponential decays with different lifetimes. These lead to an upward curvature in the decay curve (plotted as log(count rate) versus delay time), similarly to energy transfer. Using the two-state model (eq 1) to fit decays from films with donor only, one obtains a value of A2 of 0.2. If there is one single additional decay with a time constant shorter than the lifetime of the donor, this decay can

⎞ ⎛ t′ ⎞ ⎟⎟ + (1 − A 2 ) exp⎜ − ⎟ τ0 ⎠ ⎝ ⎠

(3)

A 2 (t ) − A 2,min 1 − A 2,min

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RESULTS AND DISCUSSION General. Figure 2 shows a typical data set for the dispersion with linear chains which was dried in a stream of dry air. Before

ATR and τTR are amplitude and the decay time of the fast decay, respectively. The index TR stands for “tackifying resin”. τTR was determined independently from a donor-labeled film containing tackifier. ATR was a fit parameter. Because τTR is much shorter than the lifetime of the donor τ0 (Table 3), the two contributions to the decay curves are well-separated. Values of ATR and A2 were found to be large uncorrelated. A2 can be converted to the “fraction of intermixing”, f m, defined as17

fm (t ) =

with tackifier (eq 3)

The maximum amount of FRET is assumed to be unity (meaning: is assumed to be achieved with THF films containing linear chains). The constant A2 value which was obtained from a wet, nondrying 1/ 1 (m/m) mixture of donor- and acceptor-labeled dispersions was used as A2,min because drying of the dispersion started immediately after casting and before placing it into the measurement chamber. The scattered light intensity, Iscat,raw, was detected simultaneously to the measurement of fluorescence in a second channel. A backgroundcorrected and normalized scattering intensity, Iscat,bsn (“bsn” for background-subtracted and normalized), was calculated as

NA is Avogadro’s number and cA the molar concentration of acceptor dyes surrounding the donor inside a sphere of the Förster radius RF. The parameters τ0 and 2γ were determined in separate experiments and treated as fixed, known values during fitting. To obtain the donor lifetime, a dispersion containing donor only was dried at 60 °C overnight. The function I(t′) then is a decaying exponential (no energy transfer, A2 ≡ 0), with the decay constant being equal to τ0. In order to determine the maximum FRET efficiency in a fully intermixed film (as quantified by the parameter 2γ), 3 μL of THF was dropped onto to a dry film containing linear chains labeled with both donors and acceptors 1/1 (m/m). The polymer dissolves in THF, intermixes well in solution, and then dries again as the THF evaporates. This second filmreferred to as “THF film”is considered to be fully intermixed. The decays were fitted with eq 1, where A2 was forced to 1 and 2γ was a fit parameter. τ0 was forced to the value determined on films containing donor only. Following this methodology, A2 is equal to unity for the THF film, by definition (eq 1). A2 ≡ 1 for the THF film containing linear chains is inherent to the determination of 2γ. Using eq 2, a value of around 2.9 nm was estimated for the Förster radius RF in a THF film. Given that the two-state model makes simplifying assumptions, the parameter A2 (synonymous to “the amount of FRET”) must be interpreted with some care. It might be viewed as a heuristic parameter. We have in ref 28 proposed a model-free estimator of the amount of FRET, which was the ratio ⟨tdec⟩/⟨tdec2⟩1/2 with tdec the donor lifetime in the presence of an acceptor and the weight functions in the averaging being equal to the decay curves. The comparison between A2 and this model-free parameter showed that both can serve as (heuristic) quantifiers of amount of FRET. The interpretation was not affected by which of the parameters was used. We revert to parameter A2, in order to conform with the literature.17 Decays profiles obtained from films containing a tackifying resin show an additional fast decay at short times. In order to account for the tackifier’s fluorescence, the fit function was expanded as

⎡ ⎛⎛ t ′ ⎞ t′ I(t ′) = I0⎢A 2 exp⎜⎜⎜− ⎟ − 2γ ⎢⎣ τ0 ⎝⎝ τ0 ⎠

no tackifier (eq 1)

(4) D


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Macromolecules

because the latex does not absorb light at λ = 290 nm. In phase 2, the scattering intensity of the labeled and of the unlabeled film happened to be the same. This is a coincidence with no further significance. In phase 2, there is a sharp increase in A2, coinciding with a sharp drop of scattered light, Iscat,bsn. The occurrence of such a sudden change is far from trivial. We propose an explanation in Figure 3. “Coalescence” in latex films often has a meaning slightly different from coalescence in liquid−liquid emulsions. Coalescence in latex films usually is synonymous to interdiffusion. In dense emulsions, neighboring droplets are separated by lamellae of water, similar to the bubbles in foams. These lamellae rupture when the droplets coalesce. Membrane rupture is largely absent in film formation from stiff spheres because there is a large normal pressure at the point of contact, when neighboring spheres touch. The localized force punctures the surfactant layer.12 The absence or weakness of such membranes is also supported by studies on how surfactant is dispaced from the particle−particle interfaces.35,36 For soft polymers, the surfactant is indeed expected to self-assemble into stable sheet. The liquid polymer phase may deform into polyhedra without actually coalescing (“phase 1” in Figure 3). (A few lamellae actually do break at the beginning of the experiment, as evidenced by the small increase in A2.) The membranes are stabilized by the surfactant and the pressure resulting from Darcy flow.6 In Darcy flow, one writes v = ∇pκ/ η with v the velocity of a liquid medium in a porous environment, ∇p a pressure gradient, η the liquid’s viscosity, and κ the permeability. κ has dimensions of m2 and is of the order of the cross-sectional area of the pores. If there is a flow of water from the bottom to the top, this flow creates a hydrodynamic pressure inside the system of pores. The mechanism of stabilization is lost once the wet pocket underneath the skin has dried out. The residual voids then collapse. Both scattering and the amount of FRET quickly change. Mallégol and coauthors have come to similar conclusions based on AFM images in ref 37. The authors of ref 38 have developed a similar picture when analyzing IR spectra taken on films, which dried at a temperature much above Tg. An apparent water content was derived from the integral over the water band in the IR spectra. Depending on film thickness and softness, this absorption band decreased in two separate steps. The second step was explained with a sketch similar to Figure 3. Earlier evidence for deformation preceding contact in some cases was reported in refs 39 and 40. Basing the analysis on scattering only, it would not be clear whether the scattering centers actually are lamellae or rather are cylindrical channels. The FRET data, however, decide this question. A stepwise increase in interdiffusion is only plausible, if the neighboring particles are indeed separated (by lamellae). In phase 3, the maximum observation depth of 5.5 μm is reached (see the Appendix); A2 continues to increase due to polymer interdiffusion. For coatings formulations, A2 usually enters a plateau after the film has become clear. Heating is needed to let A2 increase further in these cases.19 The continued increase of A2 observed here is explained with the low Tg. Note that the degree of intermixing, A2, never reaches value of 1. A slight decrease of A2 at the very end of the experiment is caused by bleaching of the acceptor by energy transfer. For the following data interpretation, the raw A2 values were converted into the more quantitative fraction of mixing, f m, according to eq 4.

be accounted for by fitting (see eq 3). However, such a separation is not always possible. In the experiments reported here, the origin of the fast decay is not clear at the very moment, but it also contributes to the value of A2,ini. Accounting for the additional fast decay in detail (thereby eliminating it from the analysis) is out of reach, but our results suggest that it has no significance for the FRET studies. The kinetics of A2 and Iscat,bsn show three distinct phases (dashed vertical lines in Figure 2). In phase 1, the scattering intensity gradually decreases. At the same time, the amount of FRET increases, followed by a decrease in A2 at the end of phase 1. This behavior can be understood as a consequence of skin formation, combined with the finite depth of observation. Initially, both scattering and amount of FRET report the state of the uppermost sheet of the film. The depth of information in a turbid film immediately after casting is smaller than 1 μm (see the Appendix). As reported previously,10 a fast deformation of particles creates a skin. In coatings formulations, skins often crack or warp, leading to defects on the surface of the final film. Skin formation is not a problem for PSAs because the skin is soft enough to relax mechanical stress. The amount of FRET decreases at the end of phase 1 for the reasons sketched in Figure 3. As the film becomes transparent

Figure 3. Kinetics of scattering and of the amount of FRET can only be explained if particle deformation and coalescence occur separately. Initially, attenuation is dominated by scattering. The depth of information is less than 1 μm. As particles deform into polyhedra, scattering decreases. Eventually, the depth of information increases to about 5.5 μm, dominated by absorption. Scattering does not vanish, though. It levels off to a finite value. When the wet pocket underneath the skin has dried out, the particles finally coalesce, meaning that the lamellae break up. The scattering then sharply drops, and the amount of FRET sharply increases.

at the top, the fluorescence light reaching the detector originates from greater depth than at the very beginning of the experiment. At these greater depths, interdiffusion is delayed relatively to the top. Such a decrease of A2 at the end of phase 1 was seen in most experiments. It was not observed, though, when the spot under study was located at the edge of the film. At the edge, one finds edge-in-drying rather than topdown drying. The progression of a skin from top to bottom is superseded by the progression of a lateral drying front. Contrasting to coatings formulations, such an edge-in-drying is only observed close to the edge for PSA formulations. In order to further corroborate the interpretation of phase 1 as being indicative of skin formation, a film consisting of unlabeled latex was dried (open circles in Figure 2). These data miss the drop in scattering intensity at the very beginning E


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Macromolecules Figure 4 compares film formation of latexes with linear and with covalently cross-linked chains. The first data points of f m

Figure 6. Progress of polymer interdiffusion as a function of relative humidity at different drying times. Figure 4. Kinetics of interdiffusion (a) and scattering (b) obtained from latexes consisting of linear chains and covalently cross-linked chains. Latexes were dried in a stream of dry air.

film which plasticizes the polymer chains (hydroplasticization). A faster polymer interdiffusion at higher humidities has been observed in the literature.24,25 The herein investigated polymer is a copolymer containing n-butyl acrylate and 2-ethylhexyl acrylate. According to DSC measurements performed by Tsavalas and Sundberg,41 the Tg of their corresponding homopolymers decreases when they contain water, thus their mobility is increased. Comparison of Observations Made from the Film−Air Interface and from the Film−Substrate Interface. The comparison between observation from the bottom and the top is shown in Figure 7. In all cases, phase 1 is absent in data sets

are larger than zero because A2,min was obtained from the respective wet, nondrying mixture in a sealed cuvette. Since film formation immediately starts after film casting, A2,min at the film−air interface is smaller than A2,ini. For the dispersion with linear chains A2,min is 0.6, and for the dispersion with covalently cross-linked chains it is 0.5. Phase 1 and 2 are unaffected by the polymer morphology; the differences mostly concern phase 3. In the latex with covalently cross-linked chains, polymer interdiffusion is less pronounced than for linear chains. Even the maximum value of A2 achieved by exposing the film to THF is reduced and the final film easily ruptures. These results are in accord with earlier studies.11,20 Effects of Ambient Humidity, Hydroplasticization. Figure 5 shows how variable drying speed affects the film

Figure 7. Interdiffusion and drying kinetics of a film containing linear chains, observed from the film−air and film−substrate interface at different drying speeds. Squares and darker colors are data from the film−air interface and circles and lighter colors from the film− substrate interface. (a), (c), and (e) show interdiffusion kinetics and (b), (d), and (f) show the decrease of scattering intensities for the films dried with an convectional stream of dry air, at 10% rH and at 96% rH, respectively.

Figure 5. Fraction of intermixing (a) and scattering efficiency (b) of a film containing linear chains dried at variable humidity.

formation kinetics. Increasing humidity slows down drying. The start of phase 2 is delayed when the latex is dried more slowly. To discuss the impact of the relative humidity on the film formation, plots of f m against rH at different times are shown in Figure 6. After 1000 s the latexes dried at 10% and 33% rH humidity are already in phase 3, the dispersion dried at 56% rH is in phase 2, and the dispersion at 96% rH is still in phase 1. With increasing film formation time, when all latexes are in phase 3, there is a clear evidence that the relative humidity accelerates polymer interdiffusion. This is presumably attributed to water from the surroundings being present within the

acquired form the bottom. Interdiffusion and loss of turbidity start in phase 2, following an initial period, in which f m and Iscat,bsn are largely constant. This finding supports the explanation of phase 1 in terms of a skin at the top. In all cases, f m is larger in data sets acquired from the top because drying at the top starts immediately after the film is cast. F


Article

Macromolecules

interdiffusion. These barriers disappear once the tackifier dissolves in the polymer phase. In the late stages of film formation, the tackifier speeds up interdiffusion. At around 20 000 s, the f m curves intersect. The improved mobility corresponds to what the tackifier is supposed to do: it speeds up the dynamics on long time scales, improving flow during attachment on a rough surface. This increased mobility of entire chains is also reflected in interdiffusion kinetics. In this regard, the FRET studies are consistent with results from dynamical mechanical analysis29,30 and AFM images.29

Comparing phase 3 between data acquired from the top and the bottom, the f m curves are found to intersect for the latexes which were dried very fast with a convectional stream of dry air (Figure 7a). Interdiffusion at the bottom is more pronounced than at the top. This is the consequence of the skin trapping water, which leads to hydroplasticization. Consequences of Adding a Tackifying Resin. The effects of a tackifying resin were studied on 3/1 (m/m) blends of polymer and tackifying resin. Apart from the addition of the tackifier, the experimental protocol was unchanged. Equation 2 was used for fitting, rather than eq 1. The comparison of the drying kinetics of films with and without tackifier is shown in Figure 8. In phase 2, the presence of the tackifier decreases the

■

CONCLUSIONS The combination of FRET and scattering constitutes a powerful tool to study film formation in detail. Materials designed for use as PSAs were found to dry from top to bottom. In an initial phase (phase 1), a skin forms at the film− air interface as proven by the onset of polymer intermixing. The combined information from FRET and scattering shows that there is an intermediate state of compaction, during which particles have deformed into polyhedra but are still separated by water lamellae. In phase 2, these lamellae break and particles come into contact, evidenced by rapid polymer interdiffusion and decreased scattering. In phase 3, the intensity of the scattered light stays at its minimum value, while interdiffusion continues. Covalent cross-linking does not affect phases 1 and 2 but impedes polymer interdiffusion in phase 3. Humidity has a strong impact on film formation. Increased humidity slows down drying but speeds up the long-time interdiffusion by hydroplasticization. In cases of fast drying, polymer interdiffusion at the bottom of the film was found to be faster than at the top. This is the result of skin formation. Remaining water inside the film plasticizes the chains. The addition of tackifier delays interdiffusion at early times but accelerates it at later times.

Figure 8. Comparison of interdiffusion (a) and scattering (b) of a latex composed of linear chains with and without added tackifier. Latexes were dried in a stream of dry air. The tackifier delays interdiffusion after the film has turned clear but accelerates interdiffusion later on. A2,min for the dispersion without tackifier was 0.6 and for the dispersion with tackifier 0.4.

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APPENDIX. DEPTH OF OBSERVATION It is central to the interpretation of the data that (i) the depth of observation is less than the film thickness (the latter being 175 μm in the wet state and 50 μm in the dry state), (ii) the attenuation of the beam by scattering in the wet state is more efficient than the attenuation by absorption in the dry state, and (iii) the depth of observation increases as the film becomes transparent. In the following, we support these statements with a semiquantitative argument. Extinction has two contributions, which are scattering and absorption. For the sake of an estimate of the attenuation caused by scattering, we follow van de Hulst42 and write the attenuation coefficient connected to scattering, αscat, as αscat = Figure 9. (a) Normalized emission spectrum of Phen-MMA in dimethyl sulfoxide excited at 290 nm. (b) Molar extinction coefficient of NPP-A in n-butyl acrylate. (c) g(λ) and αabs(λ).

N N σ= V V

8 ⎛ 2πnliq a ⎞ ⎜ ⎟ 3⎝ λ ⎠

4⎛

2 nliq 2 − 1 ⎞ 2 n2−1 ⎜ P ⎟ πa − ⎜n 2 + 2 nliq 2 + 2 ⎟⎠ ⎝ P

(6)

N/V is the number density of particles, σ is the absorption cross section, a is the particle radius, nliq is the refractive index of water, and nP is the refractive index of the polymer. In eq 6, independent particles with a ≪ λ are assumed. For an estimate, use the values a ≈ 60 nm, nliq ≈ 1.35, nP ≈ 1.5, λ = 290 nm, and N/V ≈ 0.3/((4π/3)a3) with 0.3 the solids content. Inserting these values leads to 1/αscat ≈ (1.7 μm)−1. Arguable, this calculation overestimates αscat because the interstices should be viewed as the scattering centers, rather than the particles. As the interstices decrease in size, αscat decreases, as well. The decay

magnitude of the step. There are two potential sources for this behavior. First, the tackifier acts as an antiplasticizer. According to DSC measurements, the Tg of the dispersions without tackifier is around −30 °C, and the Tg of the dispersions blended with tackifier is around −20 °C. Also, the tackifier initially is not mixed with the latex. It is added as separate emulsion. It dilutes the spheres and may form barriers against G


Article

Macromolecules length, αscat−1, is slightly larger for fluorescence light than for scattered light because of the increased wavelength. This is consequence of Rayleigh’s law, implicitly contained in eq 6 as the term λ4 term in the denominator. The attenuation due to absorption is calculated from Lambert−Beer’s law. For the exciting beam (λ = 290 nm), the principal source of absorption is the donor. For the emitted light, it is the acceptor. From the molar extinction coefficient of the donor, εD, and its concentration, cD, one calculates the attenuation coefficient αabs,290 as αabs,290 = εDc D ln(10)

Toronto CA) and NPP-A (97%, Sigma-Aldrich), respectively. In these solvents, the spectra match those of the corresponding dry films. εD = εPhen‑MMA at 290 nm was determined as 1.114 L/ (mol μm). The normalized emission spectrum of Phen-MMA εA(λ) = εNPP‑A(λ), g(λ), and αabs(λ) are shown in Figure 9. In the dried film from a 1/1 (m/m) blend of donor- and acceptorlabeled latexes with linear chains cD was 29 mmol/L and cA was 18.1 mmol/L. Using eq 11, one arrives at zobs,abs ≈ 5.5 μm. The spectral characteristic of the photomultiplier tube was assumed to be flat. Molar extinction coefficients and the transmission of the long-pass filter were determined with a Jasco spectrophotometer V670. The donor’s emission spectrum was determined with a Jasco spectrofluorometer FP-8500. The results from these arguments can be summarized as follows: (1) The depth of observation for scattering in the wet state is below 1 μm. It is slightly larger for fluorescence than for scattering because of Rayleigh’s law. (2) When the film−air interface is studied, the depth of observation increases as drying proceeds (for both fluorescence and scattering). (3) The final depth of observation for the fluorescence in the dry state is 5.5 μm.

(7)

Predicting the absorptive attenuation of fluorescence light is more difficult insofar as a spectrum of wavelengths is involved and that αabs depends on λ. The light reaching the detector is a weighted integral, following IDet(z) = A

∫ g(λ) exp(−αabs(λ)

2 z ) dλ

(8)

A is a prefactor covering the fluorescence quantum yield, the intensity of the exciting beam at the respective location, andif presentscattering. z is the distance to the film surface. The factor of 21/2 enters because the detectors see the sample under an angle of 45°. The spectrum of attenuation, αabs(λ), is given as αabs(λ) = ln(10)cAεA (λ)

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*E-mail: [email protected] (J.A.). ORCID

(9)

Diethelm Johannsmann: 0000-0002-8873-1742 Jörg Adams: 0000-0001-7878-2952

cA is the molar concentration of the acceptor, and εA(λ) is the molar extinction coefficient. The absorption of the donor is negligible at λ > 300 nm, and thus eq 9 only contains a contribution from the acceptor. In order to estimate a depth of observation, the probabilities for excitation and detection need to be combined. We only provide explicit equations for scattering in the initial wet state and for fluorescence in the f inal dry state. For scattering in the wet state, the depth of observation is given as zobs,scat =

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by BASF SE and Clausthal University of Technology. The authors thank Andreas Böttcher (Institute of Physical Chemistry, Clausthal University of Technology) for building the measurement chamber, B. Sc. Stephan Möbius and Udo Spuhler (BASF SE) for synthesis of the polymer dispersions, and Ulrike Koecher and Werner Bischof (Institute of Technical Chemistry, Clausthal University of Technology) for performing DSC measurements.

∫ z exp(αscat(z + 2 z)) dz 1 = αscat(1 + 2 ) ∫ exp(αscat(z + 2 z)) dz (10)

■

Inserting values, one arrives at zobs,scat ≈ 0.7 μm. The depth of observation for fluorescence in the dry state (neglecting scattering) is zobs,abs =

∫

(11)

g(λ) is a normalized weight function. It is calculated by g (λ) =

∞

0

∞

exp(− (αabs,290z + αabs(λ)z(1 +

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⎡ ∫ ∞ z exp(− (α 2 ))) dz ⎤ abs,290z + αabs(λ)z(1 + ⎥ dλ g (λ)⎢ 0 ∞ ⎢ ∫ exp(− (α 2 ))) dz ⎥⎦ abs,290z + αabs(λ)z(1 + ⎣ 0

ID(λ)T(λ) ∫

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2 ))) dz

∞

∫0 (ID(λ)T(λ) ∫0 exp(−(αabs,290z + αabs(λ)z(1 + 2 ))) dz) dλ (12)

ID(λ) is the normalized intensity of emitted light by the donor, and T(λ) is the transmittance of the long-pass filter inside the TCSPC setup. To estimate the observation depth for the final film, molar extinction coefficients and emission spectra of the monomeric dyes were determined in solution of dimethyl sulfoxide (99%, Riedel de Haën) and n-butyl acrylate (99%, purified with inhibitor removal resin, Sigma-Aldrich) for Phen-MMA (99%, H


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