# Influence of the airflow and humidity on the chain aggregation during the film-formation in a flexible waterborne polyurethane formulation

G. Ersek, R. Mehrabi Koushki, J. Scheerder, I. van Casteren, Q. Chen, D.

Hermida-Merino, G. Portale

► To cite this version:

G. Ersek, R. Mehrabi Koushki, J. Scheerder, I. van Casteren, Q. Chen, et al. Influence of the airflow and humidity on the chain aggregation during the film-formation in a flexible water-borne polyurethane formulation. Journal of Colloid and Interface Science, 2025, 678, pp.446-455. 10.1016/j.jcis.2024.09.032. hal-05122125

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Regular Article

# Influence of the airflow and humidity on the chain aggregation during the film-formation in a flexible waterborne polyurethane formulation

G. Ersek<sup>a</sup>, R. Mehrabi Koushki<sup>a</sup>, J. Scheerder<sup>b</sup>, I. van Casteren<sup>b</sup>, Q. Chen<sup>b</sup>, D. Hermida-Merino<sup>c,d</sup>, G. Portale<sup>a,*</sup>

<figure><img src="image_3.png" /></figure>

## GRAPHICAL ABSTRACT

<figure><img src="image_4.png" /></figure>

## ARTICLE INFO

**Keywords:**

Polyurethane

Waterborne colloid

GISAXS

In-situ investigation

Film formation

## ABSTRACT

**Statement of objectives:** Soft, waterborne polyurethane dispersions are indispensable components in many state-of-the-art materials, with applications ranging from binders for coatings and adhesives to matrixes for flexible devices. While the static bulk nanostructure of such systems is widely studied, the influence that environmental conditions such as relative humidity and airflow have on their film formation and phase segregation behavior in supported films is unknown.

**Experiments:** Here, we elucidate the nanostructure evolution occurring during drying of an industrially relevant, soft polyurethane, utilizing real-time, non-destructive grazing incidence X-ray scattering analysis. Using an environmental-controlled casting cell, we highlight the differences between the drying mechanism under different conditions generated by tuning the airflow and the relative humidity.

**Findings:** Our results show how the environment's relative humidity strongly influences chain aggregation and chain interdiffusion due to extended plasticization of the hard segment at high humidities, while accelerated air flows are responsible for the occurrence of (partial) skinning. Interestingly, despite changes in the chain aggregation behavior and occurrence of skinning and skin breakup during drying resulting in higher roughness at

<footnote><sup>a</sup> Physical Chemistry of Polymeric and Nanostructured Materials, Zernike Institute for Advanced Materials, University of Groningen, 9747 AG Groningen, The Netherlands</footnote>

<footnote><sup>b</sup> Covestro (Netherlands) B.V., 5145 PE Waalwijk, The Netherlands</footnote>

<footnote><sup>c</sup> Departamento de Física Aplicada, CINBIO, Universidade de Vigo, Campus Lagoas-Marcosende, E36310 Vigo, Galicia, Spain</footnote>

<footnote><sup>d</sup> DUBBLE@ESRF, Netherlands Organization for Scientific Research (NWO), BP 220, F38043 Grenoble, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: g.portale@rug.nl (G. Portale).</footnote>
the film surface, minor influence is registered on the bulk tensile properties of the films, revealing the resilient nature towards environmental conditions of these soft weakly phase segregating polyurethane systems.

# 1. Introduction

Since the discovery of polyurethane chemistry by Otto Bayer in the 1930s, polyurethanes have been proven to be extremely versatile materials [1,2] that are nowadays used for various applications such as paint primers [3,4], foams [5], adhesives [6], structural materials [7], etc. Changing the nature of the diisocyanate and diol monomers used in the synthesis, the material's characteristics can be easily tuned [8,9], allowing for the selection of distinct properties depending on the desired application.

In recent years, the application of waterborne polyurethane (WPU) formulations is increasing, as they are considered more environmentally friendly products [10–12]. In the production of a typical WPU dispersion, a prepolymer is prepared by the reaction of macrodiol, a diol/diamine that contains a stabilizing hydrophilic moiety, and an excess of diisocyanate (NCO/OH > 1) [11,13]. The prepolymer is subsequently dispersed in water to form micelle-like structures [14]. After the dispersion, the polymer chains undergo chain extension with the application of a diamine to give a high molecular weight polymer [11]. Typically, such polyurethanes have a segmented structure that consists of covalently bonded building blocks of the macrodiols and the diisocyanates/ureas [15]. Depending on the chemical properties and miscibility of these blocks, phase segregation can occur leading to a microstructure composed of nanodomains.

In addition, when WPUDs are considered, phase segregation between the commonly hydrophobic soft segments and the more hydrophilic hard segments occurs during film formation, and the extent of final phase segregation achieved is expected to be strongly influenced by the polymer dynamics and time available for the segregation process to occur [11]. Even though the static bulk nanostructure of polyurethane systems is extensively studied by various techniques [15,16], there is a lack of in-situ studies that reveal how the nanostructure develops during the drying of a PU dispersion and there is no real-time information on how environmental factors such as humidity and air flow during drying influence this process.

The film formation of waterborne latex colloids is usually described by a 4-step process [17], starting from a (viscous) liquid-like polymer dispersion (dilute phase). As water evaporates, the dispersion gets concentrated (concentration phase) and after a certain time, the particles start to deform (packing/deformation phase). The subsequent chain interpenetration (or interdiffusion) leads to the formation of a continuous film (curing phase), provided that the polymer glass transition is below the film formation temperature. The operating mechanism of the film formation process (especially the particle's behavior during the packing phase) is commonly described using the Russel-Routh model [18]. The model considers the characteristic timescales of the major processes involved in the film formation – particle deformation and particle diffusion – compared to the water evaporation rate. In particular, the $\lambda$ parameter compares the timescale of the particle deformation, while the Peclet number (Pe) compares the timescale of the particle diffusion compared to the evaporation time. Both parameters depend on the water evaporation rate ($\dot{E}$), the particle radius ($R_0$), the low shear viscosity of the polymer ($\eta_0$), the surface tension between the water and air ($\gamma_{wa}$), the initial wet layer thickness ($L_{wet}$), and the fluid phase viscosity ($\mu$). Based on the values of these parameters, various regimes can be defined, and based on the particles' Peclet number, predictions can be made regarding the vertical profile of the drying dispersion [19].

Depending on the deformation regime, the final structure and properties of the film can be vastly different, even when using identical particle architecture. For those cases when Pe > 1 (the evaporation is faster than the particle diffusion) and $\lambda < 1$ (the particle deformation is

quicker than the evaporation rate), the system can undergo skinning [19,20] which often can lead to surface defects in the dry film. Therefore, it is crucial to study and understand the latexes' behavior during drying under various environmental conditions, and careful ex-situ and in-situ analysis is necessary.

Recently, we have employed grazing-incidence small-angle X-ray scattering (GISAXS) to investigate in a non-destructive way, the film structure of polyacrylate-based coatings, allowing the study of size and distribution of coating inhomogeneities and their relationship with coating barrier properties [21].

In this contribution, we aim to expand the application of GISAXS to in-situ analysis and explore the dynamic transformations that take place at the nanoscale during film formation in soft, surfactant-free polyurethane coatings under different environmental conditions. GISAXS is the perfect tool to accomplish this task as it has been previously employed to successfully elucidate the mechanism of drying in colloidal hard systems and other nanostructured polymeric films [22,23] yet hardly used to follow in a detailed way the drying process of deformable polymer colloids.

The selected system for this study is a soft, weakly phase-segregating polyurethane based on polypropylene glycol with $M_n$ of 1000 g/mol (PPG1000) and isophorone diisocyanate (IPDI), with the use of dimethylol propionic acid (DMPA) stabilizer and hydrazine as chain extender. This system is very relevant for the industry as it is vastly used as paint primer [4,24] but used also for textile [25], medical [26], and/or film applications [27]. Yet, despite its industrial relevance, such a weakly phase-segregated PU is much less studied by X-rays, due to the low scattering signal with respect to strongly segregated PUs. Our primary objective is to analyze by real-time non-destructive investigation how the changes in the environmental factors during drying alter the film formation mechanism and the ultimate morphology of these soft, surfactant-free coatings. It must be noted, that these water-based polymer colloids are complex multicomponent systems, where several factors related to the chemistry of the system and to the environmental conditions (temperature, relative humidity, and airflow) need to be considered in order to control the drying of the cast films (i.e. the film formation process) and achieve films with long-lasting desirable properties.

# 2. Materials and methods

## 2.1. Sample specifications and casting conditions

The waterborne polyurethane colloidal dispersions were provided by Covestro (Netherlands) B.V., and were prepared by the prepolymer method [11], applying hydrazine chain extender. The polyurethane system has the following composition (see Fig. S1): polypropylene glycol (PPG, $M_n$ = 1000, soft segment), and dimethylol propionic acid (DMPA, hard segment) and isophorone diisocyanate (IPDI, hard segment), with molar fractions of $x_{PPG}:x_{DMPA}:x_{IPDI} = 0.27:0.13:0.6$ that corresponds to the following weight fractions 64.8 wt%, 4 wt%, 31.2 wt%, respectively. The applied synthetic method leads to a linear polyurethane system with no gel content [11]. The obtained polymer has a monomodal Gaussian MW distribution with MW ≫ 250.000 g/mol. The main properties of the dispersion are summarized in Table 1. The particles are charge stabilized at pH = 7.9 with triethylamine. The highest measurable shear viscosity of the bulk polymer is ~2 * 10⁵ Pa*s at 22 °C showing that the applied polyurethane formulation is ideal to cast soft coatings.

The dispersions were used for the film formation experiments starting from a solids concentration of 35.2 wt%. Macroscopically highly homogeneous films with final thicknesses in the range of 40–50 µm
<table><caption>**Table 1**
PU latex properties. The error is at the last digit of the reported value.</caption><tr><td>pH</td><td>7.9</td></tr><tr><td>Mean particle radius (DLS)</td><td>42 nm</td></tr><tr><td>Mean particle radius (SAXS)</td><td>22 nm</td></tr><tr><td>Brookfield viscosity (60 rpm)</td><td>0.14 Pa*s</td></tr><tr><td>Tg (midpoint)</td><td>-33 °C (soft segment)</td></tr></table>

depending on the drying conditions were prepared by blade coating a 120 µm thick wet layer on cleaned borosilicate glass substrate. The film formation experiments were conducted at a controlled ambient temperature of 22 °C in a custom-made casting chamber with controlled relative humidity (RH) and air flow rate (AFR) (see Fig. S2) [28]. In order to explore substantially different evaporation conditions, the relative humidity was set either to RH = 15 % or 85+ %, while the airflow was set either to AFR = 20 l/h or 100 l/h. The target RH value was achieved by bubbling air through saturated LiCl and (NH₄)₂SO₄ solutions. The used LiCl and (NH₄)₂SO₄ salts were purchased from Sigma-Aldrich and VWR respectively, and had a purity of 99 %. The RH value was monitored during the experiments using a DHT22 (AM2302) humidity sensor. Three different combinations of RH and AFR were used and defined as follows: slow evaporation rate, RH = 85+ % and AFR = 20 l/h; mid evaporation rate, RH = 85+ % and AFR = 100 l/h; fast

evaporation rate, RH = 15 % and AFR = 100 l/h. The manuscript will refer to these conditions as slow, mid, and fast drying conditions, respectively.

## 2.2. Characterization

The particle size and architecture of PU dispersion were studied by Dynamic Light Scattering (DLS), cryo-transmission Electron Microscopy (cryo-TEM), and solution Small-Angle X-ray Scattering (SAXS) experiments. DLS was performed at Covestro (Netherlands) B.V., while the Cryo-TEM was performed at the Electron Microscopy Lab of the University of Groningen, on latex with 1 wt% dispersion concentration for the SAXS and 0.5 wt% for the TEM measurements. The solution SAXS measurements were performed at the home-built MINA (Multipurpose x-ray Instrument for Nanostructural Analysis) diffractometer at the University of Groningen, equipped with a Cu rotating anode (X-ray wavelength λ = 1.5413 Å; photon energy of 8 keV) and using a hybrid pixel Dectris Pilatus 300 K detector placed ~3.1 m away from the sample. The SAXS measurements were performed both on the 35.2 wt% and 1 wt% dispersions. The stock PU dispersion was diluted with pH = 8 RO water (the pH was set by addition of cc·NH₃ solution (Sigma-Aldrich). The integration of the images was done with fit2D software. Data fitting for the 1 wt% sample was achieved using the SASfit software

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** (A) Scheme of the GISAXS geometry used at the synchrotron; (B) Typical GISAXS pattern acquired during the drying experiment; (C) Evolution of the transmitted beam intensity (I<sub>t</sub>) recorded at the photodiode (upper graph), of the diffuse GI intensity (bottom graph, thick curve), and of the specular rod intensity (bottom graph, thin curve) with time; Some selected patterns during drying are also reported. Note that the GISAXS patterns have all been acquired using a nominal incident angle of α<sub>l,n</sub> = 0.2°.</figcaption></figure>
[29] using the form factor for an ensemble of polydisperse homogeneous spherical objects.

The film formation mechanism was followed in-situ using Grazing Incidence Small Angle X-ray Scattering (GISAXS), while the final structure of the (dry) cast films was analyzed using transmission Small Angle X-ray Scattering (SAXS) on films carefully peeled from the glass substrate. The in-situ experiments were conducted at the BM26-DUBBLE beamline at the ESRF synchrotron (Grenoble), particularly optimized for in-situ and time-resolved experiments in the polymer science field (Fig. 1A) [30]. During casting, the blade was moving with a speed of ~70 mm/s. The X-ray photon energy was 12 keV (λ = 1.033 Å). The 2D GISAXS patterns were recorded on a hybrid pixel Dectris Pilatus 1 M detector. The scattering images were plotted as a function of $q_y$ and $q_z$, the modulus of the in-plane and out-of-plane scattering wavevectors, respectively:

$$q_y = \frac{2\pi}{\lambda} [\cos(\alpha_f)\sin(2\psi)]$$

$$q_z = \frac{2\pi}{\lambda} [\sin(\alpha_f) + \sin(\alpha_i)]$$

where $2\psi$ is the in-plane scattering angle in the direction parallel to the film surface, $\alpha_i$ is the incident angle of the X-Ray beam and $\alpha_f$ is the out-of-plane exit scattering angle along the surface normal measured from the film's surface.

The probed in-plane range was $q_y = \pm 0.014–0.71 \text{ nm}^{-1}$ (corresponding to a characteristic length scale range of 8.9–440 nm) and the out-of-plane one was up to $q_z = 1.48 \text{ nm}^{-1}$. Note that the GISAXS patterns presented in this manuscript are a zoomed view of the larger acquired patterns. All the acquired GISAXS patterns have been normalized for the incoming beam intensity recorded by an ionization chamber placed before the sample. A nominal incident angle of $\alpha_{in} = 0.2^\circ$ was used, which is higher than the critical angle of the material ($\alpha_c = 0.09^\circ$). Based on penetration depth calculations (see Fig. S3), we expect to obtain averaged structural information relative to the top portion of the samples with a thickness of about 8–10 µm. Total accumulation time for the in-situ measurement was 1 s per frame and an image was taken every 30 s. During the waiting time, the fast shutter was closed, limiting the X-ray exposure of the sample and avoiding unwanted radiation damage. The moment when the spreading blade stops after casting is considered as $t = 0$ min.

The BM26-DUBBLE beamline is particularly suited for these casting experiments as it allows the substrate to lie almost flat during the in-situ casting experiments. By construction, the incident beam is reflected downwards by the Si(111) mirror with an angle of 2.3 mrad, which corresponds to $0.13^\circ$ (Fig. 1A). This configuration allows the GISAXS experiments to be performed on substrates without a significant tilt (the substrate is tilted only $0.07^\circ$ during our experiments at the used incident angle $\alpha_i = 0.2^\circ$). This setup was previously utilized by Vagias et al. to study the structure of micron-thick coatings [21] and we now extend this method to in-situ analysis.

To follow the evolution in real-time during drying, integrated intensity values were used. Box sums of the intensity in various regions of interest (ROIs) were performed on the GISAXS patterns using MATLAB software (Fig. 1B). The ROI of specular rod signal ($I_{SP}$) was between $-0.035 \text{ nm}^{-1} < q_y < 0.035 \text{ nm}^{-1}$ and $0 \text{ nm}^{-1} < q_z < 1.43 \text{ nm}^{-1}$. The ROI for the diffuse GISAXS region ($I_{GI}$) was defined with the following $q$ limits: $-0.71 \text{ nm}^{-1} < q_y < -0.035 \text{ nm}^{-1}$ and $0.035 \text{ nm}^{-1} < q_y < 0.71 \text{ nm}^{-1}$ laterally and $0.20 \text{ nm}^{-1} < q_z < 0.52 \text{ nm}^{-1}$ vertically, covering the left and right part of the GISAXS pattern. Interestingly, due to the thick nature of the film, part of the direct beam traverses the top part of the sample during the drying process and the transmitted beam can be measured at the photodiode inserted in the beamstop. The change in the transmitted beam's intensity ($I_t$) over time can be used to qualitatively track the overall change in the water content, estimate the speed of

evaporation under different conditions, and monitor when the dry state is achieved (Fig. 1C).

After casting at the different conditions, the films were subjected to thermal annealing in order to induce structural relaxation and approach thermodynamic equilibrium. Dry films' transmission SAXS measurements were performed on peeled films at the previously mentioned MINA diffractometer at the University of Groningen. The SAXS profiles are obtained before and after annealing, at room temperature, using two different sample-to-detector distances (3.1 m and 28 cm) to cover a large $q$-range. The curves derived from the azimuthally averaged data acquired at multiple sample-to-detector (S-to-D) distances were merged in Origin software (OriginLabs) after rescaling the signal obtained at the larger distance with an appropriate constant so that it overlaps with the signal acquired at the short S-to-D distance. The annealing was performed at 54 °C for at least 15 h, which is well above the glass transition temperature of the soft segment but not too high to cause degradation or flow of the polymer over time.

AFM images were obtained under ambient conditions in tapping mode with a NanoScope V multimode atomic force microscope (Bruker Nano Surfaces) using silicon cantilevers with resonance frequencies of 300–400 kHz (model: TESP, Bruker Nano surfaces). The phase and height images were taken from the same area. The height and phase scales and scan area are shown on the AFM images.

The pulling tensile curves of the peeled films were recorded on an Instron tensile tester at the University of Groningen, applying a 50 %/min strain rate. The Young's moduli of the samples were determined from the initial slope of the recorded stress-strain curve. Three different samples have been measured to determine the averaged value of the elastic modulus and the error associated with it.

### 3. Results and discussion

#### 3.1. Particle characterization

The PU latex particle size and shape were first investigated in detail by microscopy and X-ray scattering. Cryo-TEM images (Fig. 2A) indicate that the particles have a spherical shape and are polydisperse in size with an average particle radius of ~23 nm (Fig. 2C). A more rigorous statistical analysis of the PU particle size and shape was performed by solution SAXS measurements.

The SAXS curve of the low concentration (1 wt%) dispersion could be fitted using a model for an ensemble of spherical particles (Eq. S2) with radius polydispersity (Fig. 2B, black circles and orange solid line). The size polydispersity of the PU particles was modelled by using the Schultz-Zimm distribution (Eq. S3).

The particles diffusion coefficient ($D_{NP}$) was found $5.15 \times 10^{-12} \text{ m}^2\text{s}^{-1}$ by DLS. This corresponds to a 42 nm hydrodynamic radius. The average value for the radius of the colloidal PU particles as found by the SAXS investigation is $22 \pm 6$ nm, in agreement with the TEM results (Fig. 2C). The particle size as derived from TEM and SAXS is smaller than the hydrodynamic radius measured by DLS as often reported for charged systems [31].

An additional SAXS investigation was conducted for the latex dispersion used for film preparation with a solids content of 35 wt% (Fig. 2B, red circles). In this case, the characteristic structure factor $S(q)$ peak related to the spatial correlation between neighboring particles is observed, with a maximum located at $q^* = 0.15 \text{ nm}^{-1}$, which means an average interparticle distance between the PU particles of $d = 2\pi/q^* \sim 42$ nm, slightly smaller than the particle diameter. This indicates that the system is packed with the initially spherical particles most likely being deformed.

#### 3.2. Macroscopic properties of the cast films

A typical optical picture of the PU film deposited using the slow
<figure><img src="image_6.png" /><figcaption>**Fig. 2.** (A) Cryo-TEM image of the PU particles (wt.% PU = 0.5 %); (B) SAXS curves and fit for the PU dispersions: black circles – signal of the dispersion with 1 wt%; black squares – signal of the dispersion with 35.2 wt%; orange line – fit of the form factor of the particle (r<sub>PU</sub> ~ 22.6 nm); (C) Comparison between the size distribution of the particles measured by TEM and by SAXS. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

conditions on a glass substrate is shown in Fig. 3A. As a result of the good film-forming properties of the studied formulation and the soft nature of the colloidal PU particles, the obtained cast films are all macroscopically homogeneous, transparent, crack-free, and with very limited edge defects. The bulk mechanical properties of the cast films are reported on Fig. 3B. It can be seen that the system's rigidity does not change dramatically with the casting conditions, even if the film prepared using the mid condition shows a slightly higher modulus. The range of the measured modulus is in agreement with previous reports for similar IPDI-based PU systems [32,33] and the small difference observed in the film rigidity may be ascribed to the combination of high humidity environment exposure and accelerated air flow rate, as it will be

<figure><img src="image_7.png" /><figcaption>**Fig. 3.** (A) A cast film; (B) Mechanical properties of the cast films (taken as the average and the spread of the measurements from three independent samples); (C) Height AFM images of the cast films; (D) Phase AFM images of the cast films.</figcaption></figure>

explained below based on the in-situ GISAXS investigation.

Conversely, the surface morphology of the cast films is strongly influenced by the casting conditions [34]. AFM analysis shows how the slow drying conditions produce the smoothest film with surface roughness of R<sub>q,slow</sub> = 0.241 nm (Fig. 3 and Fig. S4). This is a result of the slow water evaporation in humidified conditions and the high affinity of PU towards water, which leads to a high degree of water plasticization and particle coalescence. Different surface morphologies are instead recorded for the films produced with the fast and mid conditions.

For the fast-drying conditions, more pronounced valleys and protruding domains (blisters) are detected by looking at the height images (Figs. 3 and S4). The average surface roughness value measured is R<sub>q,fast</sub> = 0.563 nm. For the mid drying conditions larger aggregates are observed at the film surface, which could be due to the aggregation of some blisters, and a higher value of surface roughness of R<sub>q,mid</sub> = 0.935 nm is measured. The valleys and blisters could be the results of surface defects that are generated during faster drying as a consequence of skin formation and skin breakup, as it will be discussed below.

In addition to the topography, the phase contrast images also provide additional information on the morphology of the films. In case of mid and slow samples the brighter small domains are generally ascribed to the hard segments in the PU. It can be clearly seen that in the sample with mid drying conditions the hard segments are more aggregated which is in line with the later discussed SAXS measurements. The presence of larger aggregates in the mid-samples may look counterintuitive, but this behavior is confirmed by the in-situ GISAXS results discussed below and can be ascribed to the combination of the surface exposure to high humidity and high airflow conditions.

### 3.3. Microscopic morphology of the dry films

Due to the thick nature of the films (40–50 µm), surface AFM analysis is not able to provide a sufficiently detailed picture of the bulk film’s nanostructure. We have thus performed transmission X-ray scattering analysis which is a powerful tool to investigate the microstructure of the polyurethane samples at multiple length scales [32,33,35].

It has been often reported by combination of AFM and SAXS that PUs shows the presence of aggregates in the 10–50 nm range due to microphase separation between the hard and soft segments characterized by different solubility parameters [33]. In addition, the inhomogeneous distribution of these nanodomains has been often reported as occurring at large scales well beyond 100–200 nm [32].

Information about the interchain aggregation and segment dispersion within the film are usually derived by the portion of the SAXS curve in the low scattering region for $q < 1 \text{ nm}^{-1}$, while the portion at intermediate scattering vectors between $1 \text{ nm}^{-1} < q < 5 \text{ nm}^{-1}$ contains
information about the alternation between hard (H) and soft segments (S). In the intermediate *q*-region of the SAXS curves reported on Fig. 4, a SAXS peak located at $1.7 \text{ nm}^{-1}$ is observed and it is associated to a characteristic distance of 3.8 nm that can be assigned to the distance between the hard segment blocks along the chain, in agreement with literature [32,33]. This assignment was verified by measuring the SAXS pattern of a peeled and stretched film cast with 100 l/h air flow rate and 15 % RH (Fig. S5), where the signal is polarized along the stretching direction, i.e. in the same direction as the aligned polymeric chains. Its width and intensity are not influenced by the drying conditions.

If we consider the estimated end-to-end distance for the PPG1000 soft segments in a Gaussian conformation (17 PPG units that is a mean end-to-end distance of 2.5 nm) together with the IPDI unit length of 1.1 nm as estimated from the bond length and bond angles, the predicted distance between IPDI segments along the chain is 3.6 nm, in close agreement with the measured value of 3.8 nm. The larger measured value for the HS intrachain alternation is related to the fact that in the PU waterborne systems, hard segments can be composed of IPDI-DMPA, IPDI-DMPA-IPDI, and –(IPDI-DMPA-IPDI)$_n$-X-(IPDI-DMPA-IPDI)$_m$– (where X is the hydrazine chain extender) units, due to the synthesis of water-dispersible particles [14,36]. Such a chemical heterogeneity is very relevant for the nano-phase separation of our PU system which is strongly impacted by the drying condition.

Indeed, the SAXS signal in the low *q*-region is strongly influenced both in magnitude and shape by the drying conditions. All films show in the low *q* range below $1 \text{ nm}^{-1}$ a broad SAXS signal which overlaps to a more or less intense power law decay intensity described by the equation $I(q) = \frac{c}{q^n}$, where *c* is a constant and *n* is the power exponent. The presence of the broad signal can be graphically highlighted using a pseudo-Kratky plot representation shown in Fig. S6. For the film obtained in the mid drying conditions, the broad SAXS peak dominates, suggesting the presence of significant phase-separated nanodomains with a length scale of the order of a few tens of nm. Conversely, the SAXS peak for the slow and fast conditions appears very weak. Interestingly, the film dried in the slow conditions shows a predominance of the power law $q^{-n}$ behavior with exponent *n* = 3.7. The power law behaviour is generated by the high angle Porod contribution from large scale inhomogeneities, with average dimensions outside the measurable range ($>2\pi/q_{min}$), in line with previous reports for other IPDI-based PUs [32,33]. The presence of these larger structural features in the slow-dried film might be the results of additional chain aggregation occurring when long drying time and high humidity are applied, as it will be discussed below based on the in-situ GISAXS results. The power law exponent <4 might suggest that the interface of such large domains is diffuse.

Based on the SAXS analysis, we could summarize the nanoscale

segregation in the investigated PU films as depicted in Fig. 4B. The PU chains are composed of hard segments (IPDI/DMPA/X) spaced by soft segments (PPG1000). As these hard and soft segments exhibit weak phase separation, the presence of weak hard segment aggregation in the 10–50 nm range is observed. A further inhomogeneous distribution of the segments at the macroscopic scale leads to the formation of large (>100 nm) hard segment-rich regions.

Changes in the external drying conditions (air flow rate and the humidity) have a major influence on hard segment aggregation and the formation of these hard segment-rich regions due to the presence of the humidity-sensitive groups (i.e. DMPA groups) in the polymer chain, with a significant impact into the surface morphology, but minimal impact on the bulk mechanical properties. The SAXS results clearly show that exposing the drying wet layer to different relative humidity and different airflow conditions may trigger a more or less pronounced segment segregation as a result of an influence on the nanostructure evolution during film formation.

### 3.4. In-situ film formation studies

To shed light on the film formation mechanism in these WPUD systems and to reveal how the film nanostructure develops during drying at different conditions, in-situ GISAXS analysis was performed (Fig. 1C). GISAXS is considered a very powerful analytical technique to study the surface and bulk morphology of various types of supported films [30,37,38]. During water evaporation the intensity of the X-ray transmitted beam crossing the uppermost portion of the drying layer increases due to the reduction in the effective thickness of the absorbing material. When evaporation is complete, the transmitted beam intensity becomes constant, allowing us to retrieve the total drying time at each condition (Fig. 5, upper row). Note that we have normalized for convenience the transmitted beam intensity as $I_t = I_t(t)/I_t(t = \infty)$, such that a final value of 1 is reached at the end of the film formation process.

The overall drying time decreases when increasing the flow rate and when decreasing the RH. From the evolution of the transmitted $I_t$ signal, we measure an overall drying time of about 5 min (312 s), 25 min (1500 s), and 48 min (2850 s) for the fast, mid and slow drying conditions, respectively. Using an initial wet layer thickness of $L_{wet} = 120 \text{ µm}$ and the measured final dry film thicknesses, the evaporation rate can be calculated as $\dot{E} = \frac{L_{wet}-L_{dry}}{t_{dry}}$. The estimated evaporation rates are $2.4 * 10^{-4} \text{ mm/s}$ for the fast; $4.8 * 10^{-5} \text{ mm/s}$ for the mid; and $2.7 * 10^{-5} \text{ mm/s}$ for the slow drying conditions. Based on these values the deformation map parameters in terms of the Peclet number and the lambda parameters can be calculated [17].

<figure><img src="image_8.png" /><figcaption>Fig. 4. (A) Transmission SAXS curves of the as cast films; (B) Schematic diagram of the phase separation in the PU: along the block copolymer chain (<10 nm); between the chains – H–H aggregation (10–50 nm); Within the matrix, H-rich regions in a S-rich matrix (>100 nm).</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Fig. 5.** Upper row: normalized intensity of the transmitted beam–related to the evaporation of water with time and the film thickness reduction; Bottom row: development of the GI intensity, (thick line); development of the specular intensity (thin line) (A) 100 1/h flow rate and 15 % RH (fast), (B) 100 1/h flow rate, and 85+% RH (mid), (C) 20 1/h flow rate and 85+% RH (slow); The arrow marks the abrupt change in the specularly scattered intensity which is attributed to the skin break up.</figcaption></figure>

Using the value for the particle diffusion coefficient $D_{NP}$, the Pe numbers ($Pe_{NP} = L_{wet} \dot{E}/D_{NP}$) for the various environmental conditions are calculated to be 0.69, 1.2, and 5.6 for the slow, mid, and fast drying conditions, respectively. This indicates that the particle diffusion is slower than the evaporation speed in the case of the mid and the fast-drying conditions, suggesting inhomogeneous vertical drying [17].

To compare the speed of the particle deformation and the speed of the evaporation, the lambda parameter can be used. Within the Russel-Routh framework, we can calculate this parameter in the following way:

$$ \lambda = t_{def}/t_{evap} = \eta_0 R \dot{E} / (\gamma_{wa} L_{wet}) $$

where $\eta_0$ is the low shear polymer viscosity, R is the particle radius, $\dot{E}$ is the evaporation rate, $\gamma_{wa}$ is the water-air surface tension and $L_{wet}$ is the initial wet layer thickness. For the calculation, the same values as mentioned above for the overall drying speed at the given environmental conditions, the particle radius, and the casting temperature were used. In addition, a value of 7.2 * 10⁻² N/m was used for the air-water surface tension. The highest measurable low shear viscosity of the dry PU film was ~2 * 10⁵ Pa*s. We thus estimated the lambda parameter to be much lower than 1 (1 * 10⁻⁴, 2 * 10⁻⁵, 1 * 10⁻⁵ for the fast, mid and slow conditions respectively) for all the drying conditions. Based on the Russel-Routh deformation map [18], the main mechanism for film formation lies in between the wet sintering region (the particle coalescence happens purely in water) and the (partial) skinning region, depending on the value of the Pe number. When Pe > 1 (mid and fast conditions), the system undergoes skinning: a skin layer is formed on the liquid/air interface, trapping water under it, and influencing the drying process.

Since we are working with soft and deformable particles, during the drying process the particles pack, deform, and after coalesce with the polymer chains interdiffusing among them, transitioning from a colloidal ensemble to a continuous polymeric film. Thus, the GISAXS patterns change dramatically over time (Fig. 1C).

Immediately after casting, the GISAXS patterns are dominated by a diffuse scattering ring associated with the presence of packed colloidal PU particles. With time, the ring shifts towards larger q-values, i.e. that is a smaller interparticle distance, and slowly disappears, leaving space for the appearance of a broad Yoneda wing signal related to the formation of a swollen (semi-solid) film. The Yoneda wing appears broad and diffuse due to the thick and rough nature of the film.

The transition from a colloidal dispersion to a continuous film can be followed by the evolution of the total diffuse GI intensity. For all conditions, a comparison between the evolution of the $I_t(t)$ and the $I_{GI}(t)$ signals show a good agreement between the overall drying time and the time needed to complete the structural evolution in the films, defined as the moment when the signals do not evolve anymore with time. Since

the system transitions from particles in water to a polymeric film with (weekly segregated) hard segments dispersed in the soft PPG matrix, the electronic contrast is expected to decrease during drying, which is in agreement with the observed weakening of the $I_{GI}(t)$ signal with time.

The formation during the evaporation of a solid/air interface can be tracked by following the evolution of the specular rod $I_{SP}(t)$ intensity with time [39], which is a measure of the diffused scattering intensity generated by the formation of a rough film surface (measured in off-specular conditions). When the slowest drying conditions are applied (Fig. 5C), continuous enhancement of the $I_{SP}(t)$ signal is observed. When fast and mid conditions are used (Fig. 5A and B), the $I_{SP}(t)$ evolves slowly at the beginning and rapidly changes shortly before final drying is achieved.

Interestingly, the moment when the $I_{SP}(t)$ signal changes abruptly for the fast and mid conditions coincides with the moment when the transmitted beam $I_t(t)$ signal changes slope (i.e. evaporation rate) from a low to a higher value. In agreement with the Russel-Routh arguments discussed above, the initially slower change of the X-ray signals should be the result of skin formation in the mid and fast conditions. The abrupt change of the $I_{SP}(t)$ signal well before a dry film is formed could thus be an indication for skin rupture: skinning causes accumulation of the water under the surface and builds up stress in the material which leads to the formation of structural inhomogeneities (channels used by water to escape). This behavior seems particularly relevant for the mid conditions, where the skin breakup is more severe and introduces additional surface roughness, explaining the counterintuitive AFM topographic results illustrated above (Figs. 3C and S4).

Additional information about the details of the nanostructural evolution during the film formation process can be obtained from the change in shape of the scattering signal over time. The evolution of the in-plane horizontal $q_y$ cuts for the GISAXS signal is reported in Fig. 6. Immediately after casting, the GISAXS in-plane intensity shows a structure factor $S(q)$ peak very similar in shape and position to the one recorded by transmission SAXS for the initial concentrated dispersion, centered at $q^*$ ($t = 1$ min) = 0.15 nm⁻¹, which suggests an average distance between the particles of ~42 nm, identical to the value measured for the initial dispersion (Fig. 2B).

During the initial stage of drying, the peak position shifts gradually to larger q-values, suggesting that particles come closer together and the system is in the so-called packing phase (Fig. S7). With drying, the peak shifts gradually to a value of $q^* = 0.17$ nm⁻¹ corresponding to 37 nm particle-particle average distance, but the shape of the peak remains mostly unchanged. This distance is sensibly smaller than the minimum packing distance expected for the spherical particles with a radius of 25
<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Evolution of the in-plane *I*(*q*<sub>y</sub>) GISAXS intensity profiles during drying at the Yoneda position (GISAXS). An incident angle of α<sub>i</sub> = 0.2° > α<sub>c</sub> was used. Note that the curves are shifted vertically for clarity. Circles: GISAXS data, solid line: fit of the data; (A) 100 1/h flow rate and 15 % RH (fast), (B) 100 1/h flow rate, and 85+ % RH (mid), (C) 20 1/h flow rate and 85+ % RH (slow).</figcaption></figure>

nm, suggesting that particles are squeezed together and probably already deformed as predicted by the Russel-Routh model [18].

The duration of the first stage of drying depends on the environ-
mental conditions: about 3 min for the fast condition, 10 min for the
mid, and 15 min for the slow conditions. After the first stage of drying,
the S(q) peak does not shift further but it becomes broader and rapidly
decreases in intensity until vanishing. The broadening and vanishing of
the S(q) peak can be associated with the coalescence stage in the film
formation mechanism, where a continuous polymeric matrix is formed
at the end of drying. It should be noted that the packing and deformation
stages last very shortly for the fast-drying conditions, where the struc-
ture factor peak appears slightly different from the one measured for the
starting dispersion already after 1 min of drying.

The evolution of the in-plane GISAXS intensity cuts at low *q*-values
shows that, during the coalescence phase, regions with nanophase sep-
aration are formed, giving rise to two distinct contributions: a signal
around 0.04–0.05 *nm*<sup>-1</sup>, which corresponds to a length scale of a hun-
dred of nm; and signal located around 0.15–0.2 *nm*<sup>-1</sup>, which corre-
sponds to a length scale of ten(s) of nm. In agreement with the structural
model discussed above and based on the transmission SAXS results
presented in Fig. 4, the small structures are assigned to the local (weak)
aggregation of hard segments inside the PU film, while additional
segregation of the hard segment aggregates creates regions character-
ized by larger dimensions and a higher electron density. These larger
structures could be better observed in the GISAXS patterns due to the
higher angular resolution of the GISAXS measurements with respect to
the transmission SAXS measurements.

Quantitative analysis of the film nanostructure was thus performed
by fitting the in-plane GISAXS profiles for the dry films using a model
composed of two structural levels as described in the SI section. The
scattering from each structural level was described by using a general-
ized Guinier-Porod model (see details in SI). We attribute these struc-
tural levels to different extents of the chain/segment aggregation.
Namely, structural level *i* (small structures) refers to the local aggrega-
tion of adjacent hard segments, while structural level *i* + 1 (larger
structures) refers to the formation of larger domains due to the further
aggregation of hard segment rich domains or to the additional segre-
gation of chain segments more reach in H-bonding motifs (IPDI, DMPA).
The fitting results are summarized in Table S1. At the end of the drying
process (*t*<sub>drying</sub> ~ 70 min), the films produced show the presence of two
different structural levels.

Generally, the size of the structural level *i*, associated to the primary
hard segment aggregation, is not influenced by the drying conditions.
On the contrary, the dimensions of the larger structures associated with

the structural level *i* + 1 become significantly larger for the film dried
with low airflow and high humidity (slow conditions). While for the fast
drying the structures have size (defined as 2 * R<sub>g,i</sub>) of 20 nm and 66 nm,
dimensions of 36 nm, and 96 nm are measured for the film slowly dried
at high humidity. This trend is reasonable, as for the slow conditions the
polymer chains have more time to self-organize and the high humidity
maintains active the polymer segmental chain dynamics, especially for
the hard segments that include the hydrophilic DMPA units, which are
strongly plasticized by water [40].

The analysis of the dimensionality parameter obtained from the fit (s
parameter in Table S1) shows how the large structures are slightly
elongated (short rod-like shaped) for the fast-dried film, while they
become highly anisotropic and more lamellar-like shaped for the mid
and slow-dried films.

Interestingly, the power-law exponent which characterize the slope
of the structural level *i* + 1 in the Porod region changes from 3 in the
fast-dried film to 3.8 in the slow-dried film (m parameter in Table S1).
This suggests that when the polymer chains have little time for inter-
diffusion, the hard segments are only very weakly phase segregated and
the phase segregated domains are more defective and have rougher in-
terfaces. In general, the fact that the Porod exponent is <4 (value ex-
pected for sharp domain interface), may be a consequence of the fact
that the studied PU system is based on the IPDI and is a weekly phase
segregating system, in agreement with the literature [32,33].

It is also interesting to note that the evolution of the large structures
takes place mostly after the polymer interdiffusion stage has started
(blue curves in Fig. 6). Particularly for the mid and slow conditions, the
low angle region of the *I*(*q*<sub>y</sub>) cuts strongly evolves with time after
interdiffusion has started, while for the fast-drying conditions, the sys-
tem freezes shortly after interdiffusion.

**4. Conclusions**

In this work, we present a novel application of in-situ grazing inci-
dence small-angle X-ray scattering (GISAXS) analysis to study in a non-
destructive way and, without any perturbation, the film formation
process in thick, soft, waterborne polymer colloidal systems. By per-
forming measurements at controlled air flow and relative humidity, we
demonstrate that the film formation mechanism and the chain aggre-
gation behavior in soft, weakly phase-segregated polyurethane water-
borne systems are strongly influenced by environmental parameters.

Significantly increasing the airflow speeds up drying through
increased evaporation at the air/polymer interface. This promotes the
formation of a skin layer that slows down water evaporation and leads to
pressure build-up under it. This eventually leads to skin breakup at the later stages of drying, allowing water to escape and increasing the coating's final surface roughness. On the other side, we show how skin layer formation and breakup can be avoided by exposure to high relative humidity during drying, which allows high dynamics of polymer chains and high segmental motion during drying and enables more efficient chain interdiffusion and phase segregation. Our GISAXS results, coupled with ex-situ SAXS and AFM investigation, directly demonstrate how the combination between not too high drying rate and high relative humidity can be used to alter the films' surface structure and the extent of phase segregation in soft polyurethane systems, enabling future macroscopic property optimization of clear coatings and adhesives.

The approach presented here demonstrates how environmentally-controlled in-situ GISAXS can be considered a unique non-destructive tool to gain a deeper knowledge of the marked dependence of the film nanostructure on the environmental conditions, which in turn dictates the films' surface and bulk properties. It expands the portfolio of in-situ techniques available to study film formation in soft coatings and it is of general application. It is indeed not limited to study the formulation employed here, but it can be applied in the future to other polyurethanes: systems with stronger phase segregation behavior, systems with a more complex colloidal particle structure or presenting a more complex chemical composition and different particle nature and architectures. This will allow to isolate unique differences in the film formation mechanism and help the coating field to develop advanced and multicomponent complex formulations.

### CRediT authorship contribution statement

**G. Ersek:** Writing – review & editing, Writing – original draft, Visualization, Validation, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **R. Mehrabi Koushki:** Investigation. **J. Scheerder:** Writing – review & editing, Resources, Funding acquisition. **I. van Casteren:** Writing – review & editing, Resources. **Q. Chen:** Writing – review & editing, Resources, Investigation. **D. Hermida-Merino:** Writing – review & editing, Resources, Investigation. **G. Portale:** Writing – review & editing, Writing – original draft, Supervision, Project administration, Funding acquisition, Formal analysis, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Data availability

Data will be made available on request.

### Acknowledgements

We would like to thank Covestro (Netherlands) B.V. for their support. This research received funding from the Dutch Research Council (NWO) in the framework of the ENW PPP Fund for the top sectors and from the Ministry of Economic Affairs in the framework of the 'PPS-Toeslagregeling'. We acknowledge the European Synchrotron Radiation Facility (ESRF) for provision of synchrotron radiation facilities and we would like to thank the beamline staff for the assistance and support in using beamline BM26. EG and GP would like to thank the support of Marc C.A. Stuart (University of Groningen) for the cryo-TEM images. EG and GP would like to thank Bernhard von Vacano from BASF for his support during the data processing. EG acknowledges the support of Rick Veenstra and Wigger J. Jonker (University of Groningen) for the conceptualization and production of the environmental cell used for the film casting process. RMK acknowledges the NWO for funding her PhD

position via the project Metamaterials for Optoelectronics (MeMOE) with project number 17896 of the research programme Materialen NL.

## Appendix A. Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2024.09.032.

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