Article pubs.acs.org/Langmuir
Ultralow Stress, Thermally Stable Cross-Linked Polymer Films of Polydivinylbenzene (PDVB) Xavier Lepró,* Paul Ehrmann, Joseph Menapace, Johann Lotscher, Swanee Shin, Richard Meissner, and Salmaan Baxamusa Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, California 94550, United States S Supporting Information *
ABSTRACT: Although closely related to polystyrene, poly(divinylbenzene) (PDVB) has found limited utility due to the difficulties associated with its synthesis. As a highly crosslinked polymer, PDVB is infusible and insoluble and thus nearly impossible to shape into films by either melt or solventbased processes. Here, we report the initiated chemical vapor deposition (iCVD) of nearly stress-free, highly transparent, free-standing films of PDVB up to 25 μm thick. Films initially grow under tensile intrinsic stress but become more compressive with thickness and eventually converge to zerostress values once they reach ≥10 μm in thickness. Upon initial heating, the evaporative loss of unreacted monomer left in the polymer matrix induces between 35 and 45 MPa of tensile stress in the films. Afterward, subsequent heating cycles induce reversible stress and film expansion behaviors. We estimate the degree of cross-linking to be 44%, resulting in high thermal stability (up to 300 °C) and mechanical stiffness (Young’s modulus of 5.2 GPa). The low stress combined with high cross-linking makes iCVD PDVB an excellent candidate for protective coatings in harsh environments.
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INTRODUCTION Thermally stable, highly cross-linked polymeric thin film materials are necessary for a myriad of emergent technologies requiring lightweight, strong materials.1 Conformal polymer coatings can be used as protective barrier on substrates that are exposed to harsh operational conditions or as encapsulation for electronic components. However, despite their desirable properties, the presence of covalent bonds in cross-linked polymers restricts chain movement upon heating and limits their softening (i.e., occurrence of a glass transition, Tg). Thermoforming or injection molding after synthesis is thus difficult or impossible. This highly cross-linked network also makes them insoluble in most of solvents, precluding their casting into thin films by solution techniques. Alternatives such as in situ polymerization of the monomer on a substrate have the disadvantage of inducing surface morphology and stress generation due to monomer wetting/dewetting, surface tension, and volume shrinkage upon polymerization. These fabrication difficulties can be circumvented by avoiding the use of liquids altogether during polymerization. Initiated chemical vapor deposition (iCVD)1 achieves just that by flowing reactants in the vapor phase that adsorb to the substrate and polymerize upon reaction with a free-radical initiator. The resulting solid polymer forms directly on a substrate surface, conformally coating underlying complex geometries2,3 at the nanoscale level avoiding any dewetting and surface tension effects.4 Unlike highly energetic plasmaenhanced CVD (PECVD), iCVD is able to easily translate © XXXX American Chemical Society
established chemical polymerization mechanisms known for the liquid phase into the vapor phase, since the temperature is warm enough to promote thermolysis of the initiator but not of the monomer molecules. This avoids the well-known5−7 undesirable optical and chemical instabilities exhibited in plasma polymers due to the existence of residual radicals, dangling bonds, and photoactive defects in their structure.8 Additionally, the iCVD working pressure range of 0.01−0.3 Torr is compatible with many monomers, enabling the vaporphase synthesis of free-radical polymers, including highly crosslinked materials otherwise unattainable through standard techniques. As an adsorption-controlled process,9 substrate temperatures in iCVD are typically kept close or slightly below room temperature (i.e., 5−40 °C) during polymer deposition, which is advantageous to extend coating capabilities to a variety of materials such as paper, fabric, and other synthetic and natural polymers. iCVD is also suitable for the synthesis and casting of highly cross-linked materials. Poly(divinylbenzene) (PDVB), for example, is an infusible and insoluble homopolymer that has only been possible to prepare in either microspheres (few micrometers in diameter) by microemulsion10 or in directly casted shapes through weeks long, thermally driven bulk polymerizations in evacuated vessels.11 Given the absence of a Tg in PDVB and its lack of solubility in any known solvent, no Received: April 24, 2017 Published: May 10, 2017 A
DOI: 10.1021/acs.langmuir.7b01403 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir practical material postprocessing shaping or thin film can be produced once the polymer is synthesized. This limitation has so far hindered its application as an engineering material. Here we build upon recent reports of successful synthesis of PDVB thin films by iCVD4 and demonstrate the feasibility to produce films tens of micrometers thick that can be readily detached from their underlying substrate to form low-stress, freestanding, transparent films that are thermally stable up to 300 °C. CVD films are known to exhibit an intrinsic film stress derived from the deposition process, as is well-known for plasma polymerization and parylene coatings.12−14 Internal stresses are always present in thin films, even when not externally loaded. Since the stored strain energy increases with thickness, film stress can promote film cracking and peeling and thus are the prime limitation in the growth of very thick films. Moreover, stresses in films can induce deformation and distortion on underlying substrates,15 which in turn limits the permissible external load that can be applied on the film. Minimization of internal stresses is therefore crucial for attaining stable, high quality protective coatings on substrates and substrate-free polymeric membranes. In many applications, materials are frequently exposed to temperatures above room conditions during operation, which can lead to the generation of additional stress. While thermally induced stress is almost unavoidable in most materials, it is important that its value remain relatively low. Small induced stresses can be particularly worrisome for micro- and nanostructured materials since they could drive undesirable substrate deformation or surface rippling. Thus, ideal thin film materials would have to exhibit both low intrinsic and thermal induced irreversible stresses. To determine the suitability of PDVB thin films as an option for stand-alone and coating protective materials, we studied the overall stress evolution and properties of films exposed to cyclic thermal heating to temperatures as high as 16 °C below the onset of polymer decomposition. We found that PDVB films initially deposit under intrinsic tensile stress which asymptotes to zero with increasing thickness and that such stress increases upon film heating. We show that this stress evolution is a consequence of the unreacted monomer evaporation and discuss its effect on the other PDVB measured physicochemical properties.
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gravimetric methods using the substrate geometrical area and a value of 1.06 g/cm3 for PDVB density (Figure S1). Thick coatings (>5 μm) required several sequential depositions so that the monomer could be periodically recharged. Though these depositions stretched across several days, stress relaxation did not play a significant role due to the highly cross-linked nature of the PDVB (see Results and Discussion). Polymer Density Quantification. Geometrical volumes and masses for PDVB films of different as-deposited thicknesses (0.28, 0.44, 0.62, 0.92, 1.42, and 1.85 μm) were measured after each thermal incursion [50−280 °C] and linearly correlated by a least-squares fit (with intercept equal to zero); in all cases r2 ≥ 0.9984. Optical Transmittance of PDVB Films. UV−vis transmission spectra (Ocean Optics USB2000+XR1 spectrometer with DH-2000BAL deuterium/halogen light source) were obtained in air from a freestanding PDVB film 22.4 μm thick that was peeled from a Si wafer used as deposition substrate. Surface specular reflection was computed using the Fresnel equation (R =
(
n1 − n2 n1 + n2
2
) ) considering a normal
plane of light incidence to the polymer surface. Refraction index values as a function of wavelength (n(λ)) were obtained for PDVB by fitting ellipsometric measurements into a Cauchy dispersion expression (n(λ) = A + Bλ−2 + Cλ−4, with average measured results of A = 1.587, B = 9.0388 × 10−3, and C = 4.4799 × 10−4). Values of n(λ) for dry air were computed by the equation given in Ciddor.19 These expressions provide n values of 1.6124 and 1.0003 for PDVB and air, respectively, at λ = 633 nm. Thermal Treatment Experiments. (a) For temperatures at and below 225 °C: PDVB-coated wafers were annealed in a vacuum furnace, which was evacuated to a P = 1 Torr and subsequently taken to a pressure of 40−60 Torr by flowing dry N2 through a bleeding valve. Once the pressure stabilized, the wafers were heated to the target temperature (Ta) in 1 h and left at such value for another hour. Afterward, samples were left to cool under inert atmosphere overnight. (b) For temperatures above 225 °C: individual wafers were loaded inside a leak-tight thermal chamber at atmospheric pressure that was continually purged with N2 (200 slph) with the oxygen content continuously monitored on the effluent gas (Alpha Omega Instrument Series 2000 percent oxygen analyzer). To ensure an O2-deprived atmosphere, thermal processing of individual wafers was not started until oxygen levels reached values 400 nm) were as high as 87% as shown in Figure 5. The contribution of specular Fresnel reflections at the two interfaces with air ( 0). Thus, in the following we will adopt the convention that film stress is tensile when its value is positive (σf > 0) and compressive when negative (σf < 0). Radii of curvature of PDVB-coated substrates were measured through interferometry (see Experimental Section). As expected, a typical interferogram resulted in a spherical shape as the one shown in Figure 6B, from which R values as large as 106 mm on a wafer of 50 mm in diameter could be extracted. The film stress measured this way is the E
DOI: 10.1021/acs.langmuir.7b01403 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir We explored the dependence of intrinsic film stress with thickness on PDVB films to qualify iCVD as a suitable method to produce relatively thick protective or free-standing coatings that will not peel, crack, or undesirably modify underlying delicate substrates. Intrinsic stress values recorded for asdeposited PDVB films ranging from hundreds of nanometers to tens of micrometers are plotted in the left y-axis of Figure 7. It
tensile stress accumulated early during growth may contribute to the very low stress observed on thickest PDVB films. Further studies, especially on the early stage growth stresses, will be required to confirm the mechanisms proposed here. Because of the time required to deposit thick films, films greater than 5 μm were deposited in sequential deposition experiments. We confirmed that the intermittent deposition does not affect film stress by depositing ∼2.6 μm thick films through both methods: a single deposition and over three separate sequential depositions (each of roughly the same thickness and spaced from each other by ≥4 h). Both continuously and intermittently deposited films exhibited similar stress values after deposition, −3.8 and −3.4 MPa, respectively. Since uncertainties for all our stress measurements across different temperatures and thickness were always ≤4.1 MPa (error bars in Figure 7), both stress values are indistinguishable from each other, thus suggesting that film stress is insensitive to the intermittent deposition procedure used in our process. Thermal treatment of PDVB induces additional tensile stress as measured after the films are heated and then cooled to room temperature (at which interferometry was performed, see Experimental Section), and most of these changes were observed for treatments at temperatures below 200 °C as shown in Figure 7. Additional sample heating to 280 °C, which is just few degrees below the PDVB measured decomposition temperature, did not add considerable additional stress to the films (≤5 MPa) and intrinsic stress values asymptotically tended toward 46 MPa in thick films. To understand the cause of this film stress behavior before and after a first excursion to 200 °C, we first looked for possible changes occurring in the material around this temperature. PDVB exposed to two consecutive cyclic isothermal (1 h) treatments at 200 °C, as shown in the initial part of the thermogram in Figure 2, lost about 1.6 wt % of its initial mass. This, however, occurred only once during the first thermal cycle and mass remained constant upon subsequent cooling and reheating to 200 °C. We attribute this mass loss to the evaporation of unreacted DVB monomer inside the polymeric matrix, given that its boiling temperature is 200 °C at 1 atm41 (Figure 2B) rather than to a reaction of residual pendant vinyl groups.42 We based this observation by noting that the inflection points of both the stress and weight loss curves as a function of temperature in Figure S3 overlap at the midpoint of monomer evaporation (ca. 114 °C) as seen in Figure 2B. Furthermore, even though an exothermal reaction of unreacted CC bonds in PDVB beads has been reported between 150 and 160 °C by Li et al.,42 it does not induce a polymer weight loss when is carried out in inert atmosphere as in our case. The species evolved upon heating were identified through gas chromatography−mass spectrometry (GCMS) performed on the volatile byproducts recovered from thermally treated asproduced PDVB samples at 200 °C under N2. As depicted in Table 2, approximately 90% of these byproducts were divinylbenzene (64% meta and 13% para) or ethylvinylbenzene (6.6% meta and 5.7% para), the latter of which is a minority component in the as-supplied monomer. This confirms that the mass loss observed around 200 °C is indeed unreacted monomer. While a value of 2.2 was found for ratio of the meta- to para-DVB species in the feedstock, a ratio of 4.8 was measured for the evolved DVB species from the thermallytreated PDVB. Such difference suggests that a preferential polymerization of p-DVB over m-DVB might have occurred
Figure 7. Evolution of film stress following thermal cyclic treatments. Intrinsic film stress (on left y-axis) was obtained after subtracting the thermal mismatch stress (σT = −15.4 MPa) from the total measured stress (right y-axis). Stress measurements were taken in air, at room conditions, after the PDVB film/substrate assemblies were cooled to room temperature under inert atmosphere. Error bars are the uncertainties for each of the average intrinsic stress values reported.
is interesting to note that near zero intrinsic deposition stress values are achieved for PDVB films that are above 5 μm in thickness, which suggests that films may be grown arbitrarily thick without peeling or cracking. The highest values of stress were found in the thinnest films, but these were still below 30 MPa and comparable to other vapor-deposited cross-linked polymers such as parylene X36 or plasma polymers.12 Though the causes for the tensile stress development during the initial stages of film growth are not fully understood, similar effects have been observed on other polycrystalline and amorphous nonpolymeric films. In such systems, early deposition occurs through growth of independent islands that induce tensile stress upon island coalescence; as the film grows thicker, the additional incremental stress on the film becomes less tensile.23,37 Atomic mobility of the material during deposition, growth temperature,38 and growth rate17 are factors known to contribute to film stress in either a tensile or compressive way. For example, a low surface mobility during deposition promotes film growth under tensile stress whereas high mobilities lead to films in compressive stress.39 The evolution of film stress in our PDVB coatings exhibit a behavior qualitatively similar to the one observed in the growth of amorphous Ge (a-Ge) and Si films23,40 after isolated islands coalesce into a continuous film at about ∼4 nm. At this stage, the a-Ge film reaches a maximum tensile stress value that rapidly becomes more compressive with thickness alike to asdeposited PDVB films depicted in Figure 7. The tensile behavior we observe on the thinner PDVB films (
