Influence of Delayed, Ionic Polymer Cross-Linking on Film Formation

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

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Influence of Delayed, Ionic Polymer Cross-Linking on Film Formation Kinetics of Waterborne Adhesives Hares Wahdat,† Matthias Gerst,‡ Markus Rückel,‡ Stephan Möbius,‡ 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 impact of ionic polymer cross-linking on the film formation kinetics of waterborne dispersions designed for use as acrylic pressure-sensitive adhesives (PSAs) was studied with Förster resonance energy transfer (FRET). Aluminum acetylacetonate (Al(acac)3) was added as an ionic cross-linker to a latex with un-cross-linked chains. The polymer interdiffusion (studied with FRET) suggests that the degree of polymer cross-linking in the wet dispersion is small, as the interdiffusion kinetics is similar to that of a latex with un-crosslinked chains only. Cross-linking mainly takes place when the film dries, and it is slower than interdiffusion; thus, a homogeneous film can be formed. Ionic cross-linking in the final film through Al3+ was proven by a large gel content and an increased cohesion, the latter evaluated with tensile and tack tests. Further aspects studied include the influence of serum pH on interdiffusion and cross-linking reaction and how ionic crosslinking before film formation affects interdiffusion. polymer interdiffusion9,10 are fast, even at room temperature. However, as mentioned before, in acrylic PSAs, polymer chains have to be slightly cross-linked, either covalently or noncovalently, to achieve the desired cohesive properties.2 Covalent cross-linking of polymer chains before film formation impedes polymer interdiffusion because chains are fixed within their local network,11,12 and thus no homogeneous film is obtained from the latex.11,13 To fulfill these conflicting requirements, it is desired to cross-link the chains after interdiffusion has finished. Chains shall be mobile during interdiffusion and interconnected in the final film.4,12 Increasing cohesion in final films by cross-linking polymer chains after interdiffusion is not only done for acrylic PSAs3,14 but also for coatings.11,12,15−17 Cross-linking after interdiffusion can be achieved either by incorporating reactive groups along the polymer chains that undergo cross-linking reactions12,14,18−21 or by adding external cross-linkers into the dispersion to interconnect the polymer chains after film casting.3,12 Because of the general relevance of these latexes in terms of practical applications, theoretical models concerning the competition between polymer interdiffusion and polymer cross-linking have been developed.22,23 If the cross-linking reaction is much slower than polymer interdiffusion, a homogeneous film consisting of cross-linked polymer chains can be prepared from the latex.4,12,22,23 The paragraph above mainly discusses covalent crosslinking, which is irreversible in most cases. However, polymer

1. INTRODUCTION Polymer networks consist of polymer chains that are threedimensionally cross-linked by covalent or noncovalent bonds. Polymer cross-linking allows for modifying viscoelastic properties of polymeric materials. Tuning of viscoelastic and rheological properties is important when manufacturing pressure-sensitive adhesives (PSAs). PSAs are materials that adhere to surfaces upon application of light mechanical pressure. They can be based on acrylates having a glass transition temperature, Tg, below 0 °C (low-Tg). Their low Tg provides adhesion and tackiness. However, if the polymer chains are un-cross-linked, they do not provide the necessary cohesion desired for the specific application.1 Therefore, they are often partially cross-linked. Cross-linking decreases the tackiness and peel strength of an acrylic PSA, but increases its shear strength, which is desired if the material has to be removed from surfaces without leaving residues.1−3 Films of acrylate-based PSAs can be made from polymer solutions in organic solvents or from waterborne polymer dispersions (also termed polymer latexes).2 Preparing polymer films from waterborne dispersions is environmentally friendly because no or only small amounts of volatile organic compounds are released during film formation.4 Latex film formation consists in compaction, particle deformation, and polymer interdiffusion.4 Polymer interdiffusion is the crucial step to create a homogeneous, mechanically stable film.4 During polymer interdiffusion, particle boundaries disappear as polymer chains from neighboring particles interdiffuse and entangle with each other. Entanglements between chains create cohesion in the final film.4−6 Because of the low Tg of the acrylates used in PSA latexes, particle deformation7,8 and © XXXX American Chemical Society

Received: August 30, 2018 Revised: December 5, 2018

A

DOI: 10.1021/acs.macromol.8b01870 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

forming blend of labeled dispersions and analyzing how the curvature of the decays changes over the film formation time allow to follow the progress of polymer interdiffusion. In this paper, we investigate the influence of the ionic crosslinker Al(acac)3 on the latex film formation of industrially relevant polymer dispersions designed for use as acrylic PSAs. Al(acac)3 was added into dispersions consisting of un-crosslinked chains. Film formation, with the focus laying on polymer interdiffusion, was studied with FRET. Simultaneously, the intensity of scattered excitation light was measured to follow the state of particle deformation shortly after casting the dispersion. To explore the competition between interdiffusion and cross-linking, film formation studies of dispersions containing un-cross-linked chains, un-cross-linked chains + Al(acac)3, and covalently cross-linked chains were compared to each other. Effects of polymer interdiffusion and cross-linking on the cohesive and adhesive properties of final films were evaluated with tensile tests and flat punch tack tests. Further film formation studies concern the effects of the serum pH on interdiffusion and cross-linking reaction between un-crosslinked chains and Al(acac)3. Additionally, the effect of ionic cross-linking on polymer interdiffusion prior to film formation was studied in a reference system. Ionic polymer cross-linking during polymerization was achieved using zinc dimethacrylate. Zinc dimethacrylate was used because alumnium trimethacrylate was soluble neither in water nor in the monomer phase. In this latex, chains were ionically interconnected without undergoing a reaction with an external cross-linker after film casting.

chains can also be noncovalently cross-linked to increase cohesive properties.24,25 Noncovalent cross-linking might be reversible and can for instance be based on formation of hydrogen, coordinative (metal−ligand), or ionic bonds.24 Reversible cross-linking might be advantageous for latex film formation because chains can still interdiffuse out of their particles.4,12 This work in particular targets ionic cross-linking. Ionic cross-linking of polymers is usually based on interactions of anionic groups in the chains with metal cations. In several publications, it has been reported that ionic cross-linking of polymer chains can be reversible (i.e., transient), meaning that chains attach and detach from metal centers.26−32 In this work, the film formation of latexes designed for use as acrylic PSAs in which chains shall be ionically cross-linked in the final film is investigated. To do so, aluminum acetylacetonate, Al(acac)3, is added as an external cross-linker into a dispersion with un-cross-linked chains. Addition of metal chelates like aluminum acetylacetonate or zirconium acetylacetonate to solutions14,33,34 and aqueous dispersions3 of acrylic PSAs is usually done to increase the cohesion in the final film. For films prepared from both solutions and polymer dispersions mixed with metal chelates, it was found that the shear strength is increased and the tack is decreased, which indicates polymer cross-linking.14,34 Polymer cross-linking in these films is based on ionic interactions between carboxylate groups in the chains and multivalent metal cations. An equation proposed by Czech according to ref 34 for the crosslinking reaction between carboxylate groups in the polymer and aluminum acetylacetonate, Al(acac)3, is 3P−COO−H + Al(acac)3 ⇋ Al(P−COO)3 + 3acac−H

2. MATERIALS AND EXPERIMENTS

(I)

2.1. Materials. Polymer dispersions were synthesized via seeded emulsion polymerization under starve-fed conditions (see ref 10 for more information about the details of the synthesis). Generally, the polymer in the dispersions is a copolymer of 2-ethylhexyl acrylate, styrene, n-butyl acrylate, methyl acrylate, and methacrylic acid in a weight ratio of 59/20/15/5/1. Ionic surfactants stabilizing polymer particles against coalescence are Dowfax2A1 (Dow Chemicals) and Disponil FES 77. Polystyrene particles (30 nm hydrodynamic diameter) were used as seed and sodium peroxodisulfate as initiator. Labeled dispersions for film formation studies (properties given in Tables 1 and 2) and unlabeled dispersions for tensile and flat punch tack tests were prepared (properties given in Table 3). To study the competing effects of polymer cross-linking and polymer interdiffusion, dispersions with un-cross-linked (i.e., linear, “L”) chains were blended with aluminum acetylacetonate, Al(acac)3 (“L + Al(acac)3”). For reference studies in which chains are already cross-linked before film formation, dispersions were prepared in which 1,4-butanediol diacrylate and zinc dimethacrylate were employed as

where P is the polymer chain. According to the reaction mechanism proposed in ref 34, the carboxylate groups of polymer chains react with Al(acac)3 in a nucleophilic substitution. Acetylacetone (acac−H) is formed as a byproduct. If it can evaporate, an ionically cross-linked polymer network is obtained.34 In organic solutions of acrylics blended with Al(acac)3, stabilization by isopropyl alcohol or ethanol is necessary to prevent gelation.3,34 In polymer dispersions, the addition of a stabilizer seems not to be necessary because microgels formed within the polymer particles do not destabilize the dispersion. For acrylic PSA latexes mixed with Al(acac)3, less is known about the film formation and the degree of polymer crosslinking in the wet dispersion. It can be assumed that at least a small fraction of the polymer chains are ionically cross-linked in the wet dispersion.3,34 Still, an equilibrium is expected to be present because acetylacetone, formed after the cross-linking reaction, remains either in the particles or in the aqueous phase. By evaluating polymer interdiffusion in a film forming PSA latex blended with Al(acac)3, it can be examined if a homogeneous film composed of ionically cross-linked chains is obtained. Information on the extent of (potentially reversible) ionic polymer cross-linking in the wet dispersion and the kinetics of the cross-linking reaction can be deduced from interdiffusion studies as well. Polymer interdiffusion in film forming latexes can be explored via Förster resonance energy transfer (FRET).35 In general, two polymer dispersions with identical properties except for their labeling are prepared: one with a fluorescing donor dye as a comonomer and one with an acceptor dye as a comonomer. Capturing donor fluorescence decays of a film

Table 1. Properties of the Dispersions with Serum pH of 2 Whose Film Formation Was Investigateda dispersion

solids content [%]

dh [nm]

D-L-2 A-L-2 D-L-2 + Al(acac)3 A-L-2 + Al(acac)3 D-cov-X-2 A-cov-X-2

28 27 28 27 28 27

154 140 151 142 134 164

Mw (Mw/ Mn) [kg/mol]

gel content [%]

Tg [°C]

379 (4.5) 184 (3.8)

0 0 79 70 84 90

−30 −35 −33 −34 −31 −34

a dw/dn was not larger than 1.2 in all cases. Information on determination of the properties of the latexes is provided in section 2.2.

B


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Macromolecules

serum after polymerization reaction has finished. Investigated polymer architectures are L and L + Al(acac)3. To study how ionic cross-linking before film formation affects interdiffusion, latexes in which an equivalent amount of zinc dimethacrylate, Zn(MAA)2, instead of methacrylic acid was used as comonomer were prepared (“Zn-X”). Zn(MAA)2 had to be used because aluminum trimethacrylate was soluble neither in water nor in the monomers. Polymer chains in the wet dispersion were already ionically cross-linked by Zn2+ without undergoing a cross-linking reaction during film formation. Also in these latexes, the serum pH was neutralized to 7 after polymerization. The properties of the dispersions studied in film formation experiments are summarized in Tables 1 and 2 (details on their determination are given in section 2.2). “D” and “A” denote donorand acceptor-labeling, respectively. “L” denotes un-cross-linked (i.e., linear) chains, “cov-X-2” denotes covalent cross-linking, and “Zn-X-7” denotes ionic cross-linking by Zn2+. The numbers “2” and “7” are the pH of the serum’s dispersion. As shown in Tables 1 and 2, labeled dispersions of the same set nearly have similar properties. Gel contents of the final films from latexes mixed with Al(acac)3 are in the range of 75%. Addition of Al(acac)3 changes neither dh nor Tg. The polymer particles in the dispersions, D-Zn-X-7 and A-Zn-X-7 are slightly larger than the particles in the other dispersions. Gel contents of final films from these latexes were in the range of 72%. Molecular weight distributions of dispersions with un-cross-linked chains obtained with gel permeation chromatography (GPC) are broad. In ref 10, it has been proven that donor and acceptor are evenly distributed, independent of molecular weight. The GPC data in Table 1 differ from those reported in ref 10 because results from another GPC setup are shown in this paper. To perform tensile tests and flat punch tack tests, unlabeled dispersions with un-cross-linked chains, un-cross-linked chains + Al(acac)3, and covalently cross-linked chains were prepared. In terms of polymer architecture, these latexes have similar properties as those employed for FRET studies. Because of the industrial relevance of these dispersions, the solids content was increased to 50% and the serum pH was neutralized to 7. More properties of these dispersions are given in Table 3. Chemicals for preparing polymer dispersions were received at BASF SE, if not mentioned otherwise. Tetrahydrofuran (THF, 99.9%, inhibitor-free) for film homogenization, methyl ethyl ketone (99%), and acetylacetone (99%) were purchased from Sigma-Aldrich and used as received. Water was purified using an Arium 611 V ultrapure water system (Sartorius). 2.2. Characterization. Solids contents were determined gravimetrically. Hydrodynamic diameters, dh, were determined with dynamic light scattering (DLS, ALV/CGS-3) on dispersions diluted with ultrapure water. Scattered light (633 nm) was detected at 90°, and the temperature was 25 °C. dw/dn was not larger than 1.2 in all cases. Molecular weight distributions, Mw, Mw/Mn, were determined with gel permeation chromatography (GPC) using differential refractometer detection (Agilent 1200). The eluent was freshly distilled THF, SDV columns (PSS) were used, and the temperature was 25 °C. Polystyrene standards (PSS) were used for calibration. Gel contents were determined by swelling dried polymer films (0.5 g, 0.5−1 mm thickness, dried for 4 days at room temperature) in methyl ethyl ketone (50 g) for 4 days without stirring and collecting the insoluble fraction with a 120 μm Nylon filter (Sefar Nitex, 120 μm pore diameter). The gel content was calculated as (mgel/mfilm) × 100%, with mgel the mass of the dried, insoluble fraction collected by the filter and mfilm the mass of the dried film. Glass transition temperatures, Tg, were determined with differential scanning calorimetry (Mettler Toledo). Heating curves (−80 to +100 °C, heating rate: 10 °C/min) of latexes dried first for 4 days and then in a vacuum oven at room temperature were recorded. The temperature at the inflection point was identified with Tg. 2.3. Time-Resolved Fluorescence and Detection of Scattered Light. Time-correlated single photon counting (TCSPC, 500

Table 2. Properties of the Dispersions with Serum pH of 7 Whose Film Formation Was Investigateda dispersion

solids content [%]

dh [nm]

D-L-7 A-L-7 D-L-7 + Al(acac)3 A-L-7 + Al(acac)3 D-Zn-X-7 A-Zn-X-7

29 25 29 26 28 27

152 194 152 194 234 232

Mw (Mw/ Mn) [kg/mol]

gel content [%]

Tg [°C]

309 (4.5) 223 (4.9)

0 0 76 74 68 76

−32 −33 −32 −32 −37 −40

a dw/dn was not larger than 1.2 in all cases. Information on determination of the properties of the latexes is provided in section 2.2.

Table 3. Properties of Unlabeled Dispersions Whose Final Films Were Studied with Tensile and Flat Punch Tack Testsa dispersion un-cross-linked un-cross-linked + Al(acac)3 covalently crosslinked

solids content [%]

dh [nm]

Mw (Mw/Mn) [kg/mol]

52 52

218 218

291 (3.4)

52

216

gel content [%]

Tg [°C]

0 85

−30 −30

89

−28

a

dw/dn was not larger than 1.2 in All Cases. Serum pH was 7. Information on determination of the properties of the latexes is provided in section 2.2.

comonomers for covalent (“cov-X”) and ionic (“Zn-X”) cross-linking, respectively. More details about achieving polymer cross-linking are given in the following paragraphs. To perform FRET studies, each latex system had to be prepared twice: once with chains labeled with donor dye and once with chains labeled with an acceptor dye. Donor labeling (“D”) was achieved with 1.6 pphm (“parts per hundred monomer”, weight ratio with respect to the monomers) (9-phenanthryl)methyl methacrylate (Enamine); acceptor labeling (“A”) was achieved with 1 pphm 1-(4-nitrophenyl)-2-pyrrolidinemethyl acrylate (Enamine), which is a nonfluorescent acceptor.36 A first series of latexes were prepared to investigate the competition between polymer interdiffusion and cross-linking in a dispersion with linear chains (L) blended with aluminum acetylacetonate, Al(acac)3 (L + Al(acac)3). Polymer architectures covered are linear (L), L + Al(acac)3, and covalently cross-linked (cov-X). The dispersion L + Al(acac)3 was prepared by adding 1 pphm of Al(acac)3 to a dispersion with linear chains without stirring. After 4 days, Al(acac)3 dissolves in the latex, and a partition equilibrium of Al(acac)3 between the water phase and polymer particles is achieved. It is assumed that Al(acac)3 is mostly located inside the polymer particles due to its hydrophobicity. An exchange reaction between Al(acac)3 and Al(P-MAA)3, with PMAA the methacrylic acid units (MAA) attached to the polymer chain P, can take place (see reaction I). However, acetylacetone still remains in the dispersion, and thus an equilibrium state in which chains are expected to be at least slightly cross-linked is present.34 Covalent cross-linking (cov-X) was achieved by adding 0.5 pphm 1,4butanediol diacrylate into the monomer feed. In the dispersions of the first set, the pH of the aqueous serum was ∼2 due to the use of methacrylic acid as a comonomer and the decomposition reaction of peroxodisulfate,37 which was used as initiator. A second series of labeled latexes were prepared to study the influence of serum pH on polymer interdiffusion and cross-linking reaction. In general, neutralization of the dispersion’s serum is done to improve colloidal stability. For the dispersions investigated here, aqueous ammonia solution (10 wt %) was used to neutralize the C


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constant during the fitting procedure after being determined in separate experiments. τD, the donor’s lifetime, is obtained from a only donor-labeled film dried overnight at 60 °C (A2 ≡ 0). τD was found to be constant after the film became transparent, and only polymer interdiffusion is observed. 2γ is obtained from a film in which all chains are expected to be intermixed (A2 ≡ 1).35,38 This state was achieved by adding 3 μL of THF to a dried film of donor- and acceptor-labeled latexes with un-cross-linked chains 1/1 (m/m) and annealing it overnight at 60 °C. Films treated this way are in the following termed “THF film”. The two-state model (eq 1) makes simplifications in that it only distinguishes between interdiffused donors having intermixed with acceptors and perform FRET at the maximum rate possible (i.e., the rate in the fully intermixed film) (2γ = constant) or donors having not intermixed at all and fluoresce with their lifetime τD because they are completely isolated from acceptors.35 Intermediate situations, resulting from the fact that the parameter 2γ, which is related to the local acceptor concentration, changes during interdiffusion, are ignored. Models accounting for the nonuniform concentrations of acceptors surrounding the donor which allow for a more accurate quantification of interdiffusion based on FRET analysis have been developed by the Winnik group.39−41 Still, the two-state model used in this work to fit the donor fluorescence decays suffices to reflect the investigated structure−property relationships, which are here the effects of polymer cross-linking. A2 was converted to the “fraction of intermixing”, f m,35 according to

channels, each with a width of 0.4 ns, electronic components supplied by EG&G) was applied to record fluorescence decay curves of the donor. A pulsed light-emitting diode (LED, Picoquant) with an excitation wavelength of 290 nm, a pulse rate of 2 MHz, and 1.4 ns pulse width at half-maximum was used as excitation source. In addition to fluorescence light, scattered excitation light was measured simultaneously. Two liquid light guides (Oriel, type 77556) were used. Fluorescence photons were passed through a 360 nm bandpass filter (Schott) for the detection of the intensity of scattered excitation light, and photons were passed through a 290 nm interference filter (Schott). Light was detected with photomultiplier tubes (Hamamatsu, type R298). Measurements were performed in a home-built chamber (see ref 10 for details). In all cases, the center of the sample was studied at the film−air interface. The width of the focus of the LED was 1 mm. The temperature was 20−22 °C in all measurements. Kinetic measurements to study the interdiffusion in film forming latexes were performed by preparing blends of donor- and acceptorlabeled latexes. A 1/1 (m/m) blend of donor- and acceptor-labeled latexes was mixed for 20 s. Next, 3 μL of the prepared blend was applied onto microscope slides (spot size: 16 mm2, ellipsoidal geometry). The slide was immediately placed into the measurement chamber. The measurement was immediately started, and after the first 10 s of measurement, a stream of dry air was turned on, leading to a relative humidity of 3% within the chamber. The thickness of the initial, wet film was ∼175 μm, and that of the final, dry film was ∼50 μm. The observation depth is limited by the absorption of excitation light by the donor and absorption of emitted light by the acceptor. From static fluorescence and UV/vis absorption measurements of the monomeric dyes in solution, the observation depth after the film became transparent was estimated to be 5.5 μm and remained constant afterward (see ref 10 for details on the calculation). The accumulation time for fluorescence light was 10 s for the first 300 s of the experiments and afterward 30 s; scattered excitation light was recorded every 10 s. All measurements were performed at least twice; duplicates are not shown. Kinetic measurements of fluorescence decays in wet mixtures to obtain the initial state of intermixing in the dispersions were done by sealing 1/1 (m/m) blends of donor- and acceptor-labeled dispersions in UV-transparent quartz cells to prevent drying. To obtain constant parameters for the fitting procedure, samples were measured until 105 counts were collected at the decay’s maximum. 2.4. Data Analysis of Time-Resolved Fluorescence and Scattering Data. A Levenberg−Marquardt algorithm was applied to fit donor fluorescence decays according to the respective model. Decay curves were convoluted with the lamp signal, and the background was accounted for as well. The weighted residues, χ2, for kinetic measurements (≥104 at the decay’s maximum, 10 and 30 s accumulation time) in interdiffusion studies were never higher than 3 and for measurements in which constant fit parameters were measured (105 counts at the decay’s maximum) never higher than 8. To quantify the progress of interdiffusion in a drying film, the twostate model developed by the Winnik group35 (eq 1) was used. ÅÄÅ ÑÉÑ ijji t ′ zy ÅÅ t ′ yzz jij t ′ zyzÑÑÑÑ j Å j z j z Å I(t ′) = I0ÅÅA 2 expjjjj− zz − 2γ z + (1 − A 2 ) expjjj− zzzÑÑ jjk τD z{ ÅÅ τD zz{ k τD {ÑÑÑÖ k ÅÇ (1) where t′ is the time in nanoseconds, I0 is the intensity immediately after excitation at t′ = 0, and τD is the donor’s lifetime in nanoseconds. 2γ is defined as38 4 2γ = π 3/2NARF3cA 3

fm (t ) =

A 2 (t ) − A 2,min 1 − A 2,min

(3)

where A2(t) is the amount of FRET at the film formation time t and A2,min the minimum amount of FRET. A2,min was obtained by applying eq 1 to fit decays from wet, nondrying 1/1 (m/m) blends of donorand acceptor-labeled latexes (“DA”) which were thoroughly intermixed. Constant fit parameters and A2,min values are summarized in Table 4. A2,min values are unusually large for all dispersion systems;

Table 4. Constant Fit Parameters τD and 2γ and the Amount of FRET, A2,min, in Wet, Nondrying 1/1 (m/m) Blends of Donor- and Acceptor-Labeled Dispersions dispersion system

serum pH

τD [ns]

2γ

A2,min

DA-L-2 DA-L-2 + Al(acac)3 DA-cov-X-2 DA-L-7 DA-L-7 + Al(acac)3 DA-Zn-X-7

2

41

1.93

7

42

2.12

0.60 0.55 0.50 0.60 0.60 0.44

a value close to 0 would be expected. This issue has already been discussed in ref 10. The reason for the large value of A2,min might be water-soluble oligomers formed during emulsion polymerization42 exchanging between particles after the latexes are intermixed.10,43 Subsequent DLS measurements on the diluted dispersion blends did not show a sign of aggregation or clustering. To estimate polymer interdiffusion coefficients D from the fraction of intermixing, f m, we follow the procedure proposed by Winnik35 and use the spherical diffusion model.44

(2)

where NA is Avogadro’s number, RF the Förster radius (estimated in ref 10 to be ∼3 nm), and cA the molar acceptor concentration. A2 is the central fit parameter in eq 1 (“the amount of FRET”) and quantifies the degree of interdiffusion in the investigated film as it increases with increasing film formation time. τD and 2γ are kept

fm (t ′′) ≈ 1 −

t ″ = t − t0 D

3 4πR3C0

∫0

R

C(r , t ′′)4πr 2 dr

(4) (5)


ÄÅ É i yÑÑÑ C0 ÅÅÅ ji R + r zy zz + erfjjj R − r zzzÑÑÑ ÅÅerfjj j 2 Dt ″ zÑÑ 2 ÅÅÅÅÇ jk 2 Dt ″ z{ k {ÑÑÖ Ä É Ä É Å Ñ Å 2 l ÅÅ (R − r )2 ÑÑÑ| Dt ″ o o o ÅÅÅÅ (R − r ) ÑÑÑÑ ÅÅ− ÑÑo − − exp exp m Å Ñ Å Ñ} o Å Ñ Å o o Å Ñ Å π n ÅÇ 4Dt ″ ÑÖ 4Dt ″ ÑÑÑÖo ÅÇ ~

Article

Macromolecules C(r , t ″) = C − 0 R

regime in the low strain limit, up to ε ∼ 0.2, the elastic modulus E was calculated according to E= (6)

2 π

∫0

y

exp(−s 2) ds

(7)

f m can be fitted according to ref 35 with eq 4 to the spherical diffusion model (eq 5),44 with D being the fit parameter. r is the diffusion distance in nanometers. Initially, all chains are expected to be uniformly distributed with an initial concentration C0 inside a sphere of radius R. The average hydrodynamic radii, R = dh/2, of the spherical particles in the blend of donor- and acceptor-labeled dispersions were approximated as R and kept constant. The fraction of chains having diffused out of their particles, meaning whose location is r > R, contributes to an increase of f m. The time t″ (eq 5) is a corrected film formation time, and t0 is when the scattering intensity drops to its minimum. The obtained diffusion coefficient can be interpreted as a cumulative diffusion coefficient of all (donor- and acceptor-labeled) chains that have interdiffused up to the time t″.35 The spherical diffusion model assumes Fickian diffusion. However, the motion of polymer chains is not necessarily described by Fick’s law, but by the reptation model.45 As the herein investigated polymers have a broad molecular weight distribution, in-depth analysis regarding different time scales of polymer interdiffusion was not possible. However, fitting f m data to the spherical diffusion model allows for discussing polymer interdiffusion more quantitatively and comparing the herein obtained interdiffusion studies with literature results. In addition to donor fluorescence decays, the intensity of scattered excitation light, iscat, was determined simultaneously during film formation experiments to follow the state of particle deformation after casting the dispersion. The background-subtracted and normalized scattering intensity, Iscat, at a given film formation time t was calculated from the raw scattering intensities, iscat, following ref 10 according to Iscat(t ) =

iscat(t ) − iscat(t = ∞) iscat(t = 10 s) − iscat(t = ∞)

σtack = 4

F A

εtack =

L − L0 L0

(12)

h − h0 h0

(13)

with h the distance and h0 the film thickness which was estimated to be ∼60 μm. The adhesion energy, Wadhes, was calculated according to

Wadhes = h0

∫0

εtack,max

σtack dεtack

(14)

The average of the maximum stress, σtack,max, the strain when the stress levels off to zero, εtack,max, and the adhesion energy, Wadhes, were used for data interpretation. Standard deviations for these data were found to be very small. They might originate from a slightly nonparallel alignment of the surface of the flat punch probe and slight variations of the film thickness.

3. RESULTS AND DISCUSSION 3.1. Influence of Al(acac)3 on Polymer Interdiffusion. Figure 1 shows the interdiffusion kinetics, quantified by the fraction of intermixing, f m (a, calculated with eq 3), and intensity of scattered excitation light, Iscat (b, calculated with eq 8), of film forming dispersions (serum pH 2) with un-crosslinked chains (DA-L-2), un-cross-linked chains blended with 1 pphm Al(acac)3 (DA-L-2 + Al(acac)3), and covalently crosslinked chains (DA-cov-X-2) (see Table 1 for the properties of

(9)

with F the applied force and A the cross-sectional area of the film. The strain εtensile was calculated as εtensile =

F πd 2

The strain, εtack, was calculated using

(8)

2.5. Tensile Tests. Tensile tests were performed on films from unlabeled dispersions with un-cross-linked chains, un-cross-linked chains + 1 pphm Al(acac)3, and covalently cross-linked chains to examine the influence of cross-linking on the mechanical strength. Composition of these dispersions is similar to those whose film formation was investigated, except for their solids contents which were ∼50%. More information about the properties is given in Table 3. Tensile tests were performed using a Zwick 1465 device on films prepared by casting the respective dispersion and drying it for 10 days. Final film thicknesses were in the range 0.8−1.2 mm. Measurements were performed on strips of the polymer films (10 mm width and 40 mm length), and the testing speed was 1000 mm/min. Drying of dispersions and tensile tests were performed at 23 °C and 50% relative humidity. Forces were converted to the stress, σtensile, according to σtensile =

(11)

Five films for each polymer morphology were measured. The strain rate was around 0.4 Hz (as calculated by dividing the testing speed by L0). The average values of the maximum stress and strain, σmax,tensile and εmax,tensile, respectively, the stress and strain at fracture, σfrac,tensile and εfrac,tensile, respectively, and the elastic moduli E were used for data interpretation. Standard deviations for these data were also provided. 2.6. Flat Punch Tack Tests. Flat punch tack tests were performed on final films from unlabeled dispersions (properties of dispersions given in Table 3) to examine the influence of cross-linking on the performance regarding the application as a PSA. Films were spread on glass plates using a film applicator (120 μm gate height) and were first dried for 4 days at 23 °C and 50% rH and then annealed at 50 °C in an oven overnight. According to the solids content of the dispersions (∼50%), thicknesses of the final films were estimated to be around 60 μm. To the eye, the surface of the film was flat and homogeneous in all cases; therefore, it was assumed that the thickness of the film was almost even over the entire surface. Flat punch tack tests were performed using a TA.XT plus Texture Analyzer (Stable Micro Systems) at 23 °C and 50% rH. A parameter set commonly used in the literature was applied.1 Contact pressure was 1 MPa, contact time was 10 s, compression speed was 30 μm/s, and debonding speed was 10 μm/s. A flat, cylindrical metal stamp with a diameter d = 2 mm was used as a probe. The curves have a good reproducibility, and the data quality suffices for the comparative study regarding the different polymer architectures in the films. However, a slightly nonparallel alignment of the probe to the surface of the polymer film (resulting in a contact are different from the one calculated below) cannot be ruled out. Force−distance curves were converted to stress−strain curves. Two films for each latex system were prepared; five spots of each film were measured. To calculate the stress σtack, the force F was divided by maximum contact area possible, which is the area of the circular stamp, according to

with erf the error function: erf =

σtensile εtensile

(10)

with L the length of the stretched film and L0 the initial length. Stress−strain curves were recorded until εtensile = 15. For the linear E


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when acetylacetone, the byproduct, can evaporate (see ref 34 and reaction I). f m data suggest that polymer interdiffusion is faster than polymer cross-linking. Ionic cross-linking in the final films was proven by a gel content of ∼75% in methyl ethyl ketone (see Table 1). However, in acetylacetone, the gel was completely soluble, which is in accord with the equilibrium proposed in ref 34 and reaction I. In the case of covalent crosslinking (DA-cov-X-2), almost no interdiffusion occurred as most of the chains are fixed within their particles. After intermixing films with THF and annealing them at 60 °C overnight (“THF film” in Figure 1), films have f m values of 1, 0.87, and 0.44 for DA-L-2, DA-L-2 + Al(acac)3, and DA-cov-X2, respectively. A correlation between polymer interdiffusion studies and the mechanical properties of the final films is done in section 3.4. To interpret polymer interdiffusion in the film forming dispersions in more detail, selected f m data covering the entire film formation time shown in Figure 1a were fitted to the spherical diffusion model according to eq 4 to obtain apparent polymer diffusion coefficients D. Results for the dispersions DA-L-2, DA-L-2 + Al(acac)3, and DA-cov-X-2 are shown in Figure 2. The time t0 is when the film turns clear and only polymer interdiffusion is observed, which is at ∼600 s.

Figure 1. Interdiffusion (a) and scattering data (b) for latexes with a serum pH of 2. DA-L-2: un-cross-linked chains; DA-L-2 + Al(acac)3: un-cross-linked chains mixed with 1 pphm Al(acac)3; DA-cov-X-2: covalently cross-linked chains. Data sets for DA-L-2 and DA-cov-X-2 were already presented in ref 10.

the dispersions). Data for DA-L-2 and DA-cov-X-2 were recently presented in ref 10. All f m data start at values higher than zero because skin formation starts immediately after film casting,10 and thus it was necessary to obtain the A2,min value from the respective wet, nondrying latex blend sealed in a quartz cell to calculate f m (see Table 4). In our previous paper,10 we interpret the time evolvement of the interdiffusion and scattering data, when the film is still wet and scatters light (before the solid line in Figure 1 at ca. 600 s) as a consequence of heterogeneous top-down drying combined with an increase of the observation depth. Because the states of film formation up to the solid line in Figure 1 are unaffected by the polymer architecture, the arguments given in ref 10 are only shortly summarized. It has been distinguished between three phases. Phase 1, up to 400 s (dotted line in Figure 1), indicated by the slight increase of f m, is attributed to skin formation starting immediately after casting the film. Iscat, the intensity of scattered excitation light, decreases because it is attenuated by absorption through the donor dyes in the deformed layers. It is assumed that the soft particles under the skin deform but are still separated by water lamellae. Phase 2, from around 400−600 s (solid line in Figure 1), which is indicated by the strong increase of f m and decrease of Iscat to zero, is when the water lamellae dry out, and the regions below the skin turn clear as the deformed particles under the skin come into contact. This is also the time when the maximum observation depth of ca. 5.5 μm is reached and remains constant for the rest of the experiment. Note that the final film thickness is ca. 50 μm. Afterward, only polymer interdiffusion (phase 3, starting from the solid line in Figure 1), is observed as indicated by the continuous increase of f m, which is affected by the polymer architecture and discussed in the following. The dispersion with un-cross-linked chains (DA-L-2) and the dispersion with un-cross-linked chains blended with the ionic cross-linker Al(acac)3 (DA-L-2 + Al(acac)3) have similar interdiffusion kinetics until 5000 s (see Figure 1a). Afterward, f m remains constant at a value of ca. 0.7 for DA-L-2 + Al(acac)3, whereas it still increases for DA-L-2. The interdiffusion studies imply that most of the chains in DA-L2 + Al(acac)3 are un-cross-linked in the wet dispersion. The cross-linking reaction between carboxylate groups in the polymer chains and Al(acac)3 mainly occurs during drying

Figure 2. Evolution of polymer diffusion coefficients D over the corrected film formation time t″, t0 ∼ 600 s.

The plot of D versus the corrected film formation time t″ reveals a linear decrease of D in a double-logarithmic plot for all dispersions. D values for DA-L-2 and DA-L-2 + Al(acac)3 start at values around 101−102 nm2/s and decrease to 10−1 nm2/s after 10000 s of film formation time. This decrease can be explained by the broad molecular weight distribution of the polymers. At early times the faster, smaller chains interdiffuse and contribute to an increase of the amount of FRET, while at later times, the slower, longer chains contribute to the amount of FRET as well. Such a decrease of the polymer interdiffusion coefficient along the film formation time due to the large polydispersity of the polymer chains has also been described in the literature.35,41,46−49 Taking the difference of ca. 50 °C between ambient temperature during interdiffusion and the polymer’s Tg into account, the obtained diffusion coefficients are in a similar range compared to interdiffusion studies performed on latexes of poly(n-butyl methacrylate), a polymer commonly used in coatings formulations, as reported in ref 35 (FRET), ref 50 (small-angle neutron scattering), and ref 46 (excimer formation). The similarity between the diffusion data F


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interdiffusion is slower for the dispersion with a serum pH of 7 (DA-L-7). This is attributed to the conversion of methacrylic acid units located at the surface of the polymer particles51,52 into their ammonia salts due to the neutralization of the serum. This results in a more hydrophilic layer which delays polymer interdiffusion.53 Still, at later times, f m values become eventually similar. In the case of dispersions with un-cross-linked chains + Al(acac)3, f m data shown in Figure 4b, the same argument is valid. However, the difference in the interdiffusion kinetics is larger than expected in comparison to the data in Figure 4a. The much slower interdiffusion in the latex with serum pH 7 (DA-L-7 + Al(acac)3) can be traced back to a faster crosslinking reaction between polymer chains and Al(acac)3 as more carboxylate groups in the chains are present due to the neutralization of the serum (see ref 34 and reaction I). 3.3. Influence of Ionic Cross-Linking before Film Formation. Polymer interdiffusion in a latex with ionically precross-linked chains was compared to interdiffusion in a latex with un-cross-linked chains + Al(acac)3. Ionic cross-linking before film formation was achieved by employing zinc dimethacrylate, Zn(MAA)2, as a comonomer instead of methacrylic acid. Zn(MAA)2 was used instead of aluminum trimethacrylate as the latter was soluble neither in water nor in organic solvents. In this dispersions, chains are already ionically cross-linked without undergoing a reaction with an external cross-linker. Film formation kinetics of the dispersions which all have a neutral serum pH of 7 (see Table 2) containing uncross-linked chains (DA-L-7), un-cross-linked chains blended with Al(acac)3 (DA-L-7 + Al(acac)3, 75% gel content), and chains whose methacrylic acid units in the wet dispersions are cross-linked by Zn2+ (DA-Zn-X-7, 72% gel content) are shown in Figure 5.

of DA-L-2 and DA-L-2 + Al(acac)3 suggests a slow polymer cross-linking compared to a fast polymer interdiffusion, as already discussed. Covalent cross-linking prior to film formation hinders polymer interdiffusion, as shown by the small D and f m values for DA-cov-X-2. To compare how polymer interdiffusion changes over the degree of intermixing, plots of D versus f m are shown in Figure 3.

Figure 3. Diffusion coefficients against the fraction of intermixing for DA-L-2 and DA-L-2 + Al(acac)3. Data until t″ = 11000 s are shown, t0 ∼ 600 s.

The polymer interdiffusion kinetics are similar for DA-L-2 and DA-L-2 + Al(acac)3 as indicated by the same range of the f m and D values. Chains in the latex containing Al(acac)3 are un-cross-linked until f m reaches its limit of ∼0.7. Data of the latex with covalently cross-linked chains are not shown in Figure 3 because f m stays constant at 0.2 over time as no significant polymer interdiffusion takes place. 3.2. Influence of Serum pH. Interdiffusion kinetics of dispersions with acidic (pH 2, see Table 1) and neutral serum (pH 7, see Table 2) are shown in Figure 7. Film formation kinetics including the scattering data of the latexes with serum pH 7 are discussed in section 3.3. In Figure 4a, interdiffusion kinetics of dispersions with uncross-linked chains with different serum pH are shown. At early times, starting from the strong increase of f m at ca. 400 s,

Figure 5. Interdiffusion (a) and scattering data (b) for latexes with a serum pH of 7. DA-L-7: un-cross-linked chains; DA-L-7 + Al(acac)3: un-cross-linked chains + 1 pphm Al(acac)3; DA-Zn-X-7: ionically precross-linked chains. The solid line indicates the time after which only polymer interdiffusion (phase 3) is observed.

Interdiffusion (Figure 5a) and scattering data (Figure 5b) reveal that the initial phases until 600 s in which the film is still wet are not affected by polymer morphology. A possible explanation for these initial phases has been given in section 3.1. The increase of f m due to polymer interdiffusion in Figure 5a after the film has become transparent at ∼600 s, indicated by the solid line in Figure 5, is discussed in the following.

Figure 4. Impact of serum pH on polymer interdiffusion in dispersions with (a) un-cross-linked chains (black squares: DA-L-2; gray rhombuses: DA-L-7) and (b) of un-cross-linked chains with the addition of 1 pphm Al(acac)3 (red circles: DA-L-2 + Al(acac)3; blue triangles: DA-L-7 + Al(acac)3). Data in panel (a), pH 2 were already presented in ref 10. G


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Macromolecules Polymer interdiffusion is impeded for Zn-X-7 because chains have been ionically cross-linked prior to film formation. However, as indicated by the continuous increase of f m for ZnX-7, interdiffusion is not hindered, but rather slowed down. This can be interpreted as a sign that chains can attach and detach from Zn2+ centers, implying that polymer chains are reversibly cross-linked, as it has been reported in ref 31. THF films of the dispersions DA-L-7, DA-L-7 + Al(acac)3, and DAZn-X-7 have f m values of around 1, 0.80, and 0.75, respectively. Plots of D versus f m are provided in Figure 6.

Figure 7. Exemplary stress−strain curves of final films with un-crosslinked, chains ionically cross-linked by Al(acac)3 and covalently crosslinked chains obtained with tensile (a) and flat punch tack tests (b).

results with the interdiffusion studies are demonstrated in the following. In Figure 7a, it is revealed that the cohesion in the film with un-cross-linked chains is provided mainly by entanglements of polymer chains, and it is too weak to induce strain hardening. The film from the dispersion with un-cross-linked chains blended with Al(acac)3 (“un-cross-linked + Al(acac)3” in Figure 7) consists of ionically cross-linked chains as it shows fracture and strong strain hardening which is characteristic for a tight, interconnected polymer network.1,55 The film with covalently cross-linked chains fractures at almost the same stress but shows a much weaker strain hardening compared to un-cross-linked + Al(acac)3 (see Table 5). The differences between the cohesion in these films can be explained by taking the interdiffusion studies performed on dispersions with the same polymer architectures (see Figure 1a) into account. In the case of dispersions with un-crosslinked chains (DA-L-2 in Figure 1a) and un-cross-linked chains + Al(acac)3 (DA-L-2+Al(acac)3 in Figure 1a), interdiffusion is not retarded, and thus a homogeneous film, in which particle boundaries have disappeared, is obtained. Polymer crosslinking by Al(acac)3, which is slower than interdiffusion, further increases the cohesion in the final film significantly. In the case of the latex with covalently cross-linked chains interdiffusion is strongly hindered (see cov-X-2 in Figure 1), and particle boundaries remain, resulting in a final film with a lower value of σfrac,tensile11,13 compared to un-cross-linked + Al(acac)3. Employing Al(acac)3 as a cross-linking agent for uncross-linked chains in PSA latexes allows for both sufficient polymer interdiffusion, contrary to films from latexes with covalently cross-linked chains, and strong strain hardening, contrary to films from latexes with un-cross-linked chains only. Tack tests (Figure 7b and Table 6) reveal that the tackiness of the final film of un-cross-linked chains + Al(acac)3 is highly decreased compared to the film with un-cross-linked chains, which implies a high degree of ionic polymer cross-linking. The film with un-cross-linked chains shows cohesive failure, while the film from un-cross-linked + Al(acac)3 shows adhesive failure. A decreased tack and increased cohesion in films from acrylic PSA dispersion blended with Al(acac)3 have also been reported in the literature.3 Here, the amount of Al(acac)3 in the dispersion is 1 pphm, resulting in a gel content of 85%. In general, slight cross-linking of the chains, achieved by

Figure 6. Polymer interdiffusion coefficients against the fraction of intermixing for DA-L-7, DA-L-7 + Al(acac)3, and DA-Zn-X-7. Data until t″ = 11000 s are shown, t0 ∼ 600 s.

The values of polymer diffusion coefficients, D, and the shape of their decrease along the fraction of intermixing, f m, are similar for all latexes. However, curves are shifted toward smaller f m values for DA-L-7 + Al(acac)3 and DA-Zn-X-7, especially in the latter case. For the dispersions DA-Zn-X-7, it can be attributed to the ionic cross-linking of chains in the polymer particles prior to film formation. Upon comparison of polymer interdiffusion between DA-L-7 and the DA-L-7 + Al(acac)3, intermixing of polymer chains is less pronounced in the latter case due to the polymer cross-linking reaction being faster, as discussed in section 3.2. 3.4. Tensile and Flat Punch Tack Tests. To correlate the cohesive and adhesive properties of the final films with interdiffusion studies shown in Figure 1a, comparative studies consisting of tensile and flat punch tack tests were performed on films prepared from unlabeled latexes (see Figure 7). Films with un-cross-linked chains (0% gel content), un-cross-linked chains + Al(acac)3 (85% gel content), and covalently crosslinked chains (89% gel content) were investigated (see Table 3 for more information about the composition of these latexes). Exemplary plots of stress, σ, against strain, ε, for tensile and tack tests are shown in Figures 7a and 7b, respectively. Tensile tests (Figure 7a) reveal that the film consisting of un-cross-linked chains does not fracture and flows until the end of the experiment, contrary to the films consisting of both ionically and covalently cross-linked chains. The E moduli in the linear regime of the low strain limit are all slightly above the Dahlquist criterion1,54 which is E ≤ 0.1 MPa at a strain rate of 1 Hz (here, the strain rate is 0.4 Hz). This might be attributed to the formation of entanglements of the un-crosslinked chains within the films.55,56 The influence of polymer cross-linking on the tensile tests and a comparison of the H


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Table 5. Results of Tensile Tests Shown in Figure 7a: σmax,tensile and εmax,tensile Are the Maximum Stress and Strain, Respectively; σfrac,tensile and εfrac,tensile Are the Stress and Strain at Fracture, Respectively; and E is the Elastic Modulus in the Low Strain Limit (Numbers in Parentheses Are Standard Deviations) polymer architecture

εmax,tensile

σmax,tensile [MPa]

εfrac,tensile

σfrac,tensile [MPA]

E [MPa]

un-cross-linked un-cross-linked + Al(acac)3 covalently cross-linked

0.88 (±0.03) 0.65 (±0.03) 0.45 (±0.02)

0.14 (±0.01) 1.12 (±0.11) 0.39 (±0.02)

0.65 (±0.03) 0.59 (±0.04)

1.12 (±0.11) 0.21 (±0.08)

0.22 (±0.01) 0.28 (±0.01) 0.32 (±0.02)

Table 6. Results of Flat Punch Tack Tests Shown in Figure 7b: σmax,tack Is the Maximum Stress, εmax,tack Is the Strain When the Stress Levels off to Zero, and Wadhes Is the Adhesion Energy (Numbers in Parentheses Are Standard Deviations) εmax,tack

σmax,tack [MPa]

Wadhes [J/m2]

30 (±1.8) 0.80 (±0.05)

0.49 (±0.06) 0.35 (±0.02)

100 (±6.6) 6.76 (±0.55)

0.80 (±0.08)

0.44 (±0.03)

6.91 (±0.55)

polymer architecture un-cross-linked un-cross-linked + Al(acac)3 covalently cross-linked

ACKNOWLEDGMENTS

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REFERENCES

The authors thank Andreas Böttcher (Institute of Physical Chemistry, Clausthal University of Technology) for manufacturing the chamber for TCSPC and light scattering measurements, Martina Heinz (Institute of Technical Chemistry, Clausthal University of Technology) for performing GPC measurements, Ulrike Koecher, Werner Bischof, and Martin Schwedes (Institute of Technical Chemistry, Clausthal University of Technology) for performing DSC measurements, Udo Spuhler (BASF SE) for synthesis and analysis of polymer dispersions, and Dr. Rui de Oliveira (BASF SE) for performing tensile tests.

decreasing the amount of cross-linker, can increase the tack and enable fibrillation.1 However, for Al(acac)3 in particular, the tack was found to decrease also at concentrations below 1 pphm.3 Combining the results of the tack measurement in Figure 7b with the interdiffusion studies (see Figure 1a) confirms the formation of a homogeneous film with a high cross-linking degree. The tack of the film from the dispersion with covalently cross-linked chains is similar to that of uncross-linked + Al(acac)3. However, as indicated by the interdiffusion studies (see Figure 1a) and tensile tests (see Figure 7a), the final film from the latex with covalently crosslinked chains is less homogeneous and fractures at a lower stress.

(1) Creton, C.; Ciccotti, M. Fracture and Adhesion of soft materials: a review. Rep. Prog. Phys. 2016, 79, 046601. (2) Jovanović, R.; Dubé, M. A. Emulsion-based Pressure-Sensitive Adhesives: A Review. J. Macromol. Sci., Polym. Rev. 2004, 44, 1−51. (3) Czech, Z. Crosslinking of pressure sensitive adhesive based on water-borne acrylate. Polym. Int. 2003, 52, 347−357. (4) Keddie, J. L.; Routh, A. F. In Fundamentals of Latex Film Formation, 1st ed.; Pasch, H., Alig, I., Janca, J., Kulicke, W.-M., Eds.; Springer Science+Business Media LLC: Dordrecht, 2010; pp 11, 175−179. (5) Voyutski, S. S. Amendment to the Papers by Bradford, Brown and Co-Workers: “Concerning Mechanisms of Film Formation from High Polymer Dispersions”. J. Polym. Sci. 1958, 32, 528−530. (6) Vanderhoff, J. W. Mechanism of Film Formation of Latices. Br. Polym. J. 1970, 2, 161−173. (7) Routh, A. F.; Russel, W. B. Deformation Mechanisms during Latex Film Formation: Experimental Evidence. Ind. Eng. Chem. Res. 2001, 40, 4302−4308. (8) Carter, F. T.; Kowalczyk, R. M.; Millichamp, I.; Chainey, M.; Keddie, J. L. Correlating Particle Deformation with Water Concentration Profiles during Latex Film Formation: Reasons that Softer Latex Films Take Longer to Dry. Langmuir 2014, 30, 9672− 9681. (9) Feng, J.; Pham, H.; Stoeva, V.; Winnik, M. A. Polymer Diffusion in Latex Films at Ambient Temperature. J. Polym. Sci., Part B: Polym. Phys. 1998, 36, 1129−1136. (10) Wahdat, H.; Hirth, C.; Johannsmann, D.; Gerst, M.; Rückel, M.; Adams, J. Film Formation of Pressure-Sensitive Adhesives (PSAs) Studied with Förster Resonance Energy Transfer (FRET) and Scattering Intensity. Macromolecules 2018, 51, 4718−4726. (11) Zosel, A.; Ley, G. Influence of crosslinking on structure, mechanical properties and strength of latex films. Macromolecules 1993, 26, 2222−2227. (12) Taylor, J. W.; Winnik, M. A. Functional Latex and Thermoset Films. JCT Research 2004, 1, 163−190. (13) Pinenq, P.; Winnik, M. A.; Ernst, B.; Juhué, D. Polymer Diffusion and Mechanical Properties of Films Prepared from Crosslinked Latex Particles. J. Coat. Technol. 2000, 72, 45−61. (14) Tobing, S. D.; Klein, A. Molecular Parameters and Their Relation to the Adhesive Performance of Acrylic Pressure-Sensitive Adhesives. II. Effects of Crosslinking. J. Appl. Polym. Sci. 2001, 79, 2558−2564.

4. CONCLUSIONS Blending waterborne dispersions designed for use as acrylic PSAs with the ionic cross-linking agent Al(acac)3 allows for the preparation of homogeneous films with ionically cross-linked chains. Polymer interdiffusion studies suggest that most of the chains in the wet latex are un-cross-linked and that the crosslinking reaction, mainly taking place after film casting, is slower than polymer interdiffusion. After a certain film formation time, effects of ionic cross-linking become noticeable as interdiffusion becomes slower. Ionic cross-linking in the final film was proven by a large gel content and a decreased tack. Neutralizing the serum pH from 2 to 7 delays interdiffusion and accelerates the cross-linking reaction between the polymer chains and Al(acac)3. Tensile tests on the final films show that films from a latex with un-cross-linked chains blended with Al(acac)3 have a better mechanical stability than films consisting of covalently cross-linked chains with the same gel content. Ionic cross-linking of polymer chains before film formation still allows for interdiffusion, but slows it down. In contrast, covalent cross-linking entirely hinders interdiffusion.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jörg Adams: 0000-0001-7878-2952 Notes

The authors declare no competing financial interest. I


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