Because of the rapid progress in nanotechnology, particularly the recent development of bottom–up chemical methods and top–down physical methods, the long-held desire to fashion materials of the smallest possible size, possibly even of molecular dimensions, seems to be in sight. However, we have not arrived at a stage at which highly functionalized tiny devices can be routinely produced by nanotechnology. One of the most difficult problems to solve is the lack of an understanding of how such minute devices can be assembled and combined with one another. Of course, it is technically difficult to paste adhesives homogeneously onto such nanopieces. Even if this should become possible, adhesives may act as deleterious impurities in such a nanovolume system and thus should not be used. Therefore, it is key for the success of nanotechnology to develop novel adhesion techniques without the use of adhesives. Our strategy for achieving this objective is to use enhanced mobility at the surface to promote adhesion between polymers.
It is widely accepted that adhesion phenomena in polymers are closely related to their viscoelastic properties.1, 2 In particular, in the case of a miscible polymer/polymer system, interdiffusion between the layers is one of the factors responsible for the adhesion strength.3–5 Prud'homme and coworkers6–8 have systematically investigated self-welding between polystyrene (PS) layers, that is, a PS bilayer, with several types of mechanical tests. They have revealed that the adhesion strength increases with time even at a temperature below the bulk glass-transition temperature (T); this means that interfacial PS chains, which originally exist on the PS surfaces, can diffuse at a temperature lower than T. This finding is essentially consistent with our previous conclusions; that is, the surface mobility in PS films is enhanced in comparison with the corresponding mobility in the bulk.9–11 However, to what depth range welding occurs at temperatures below T is still an open question.
In this article, we examine the relation be tween the adhesion strength and interfacial width at a temperature below T by a conventional lap-shear test in conjunction with dynamic secondary-ion mass spectrometry (DSIMS). At these temperatures, adhesion takes place over an interfacial thickness (d) on the nanometer scale. Then, we discuss a chain-end effect on the adhesion strength via enhanced surface mobility.
The samples used in this study were monodisperse PS and deuterated polystyrene (dPS) that were terminated at one end by a sec-butyl group and at the other by a proton. Also, polystyrene terminated by fluoroalkyl end groups [α,ω-PS(Rf)2], synthesized by a living anionic polymerization with potassium naphthalene as an initiator and (tridecafluoro-1,1,2,2,-tetrahydrooctyl)dimethylchlorosilane as a terminator, was used.12 Table 1 shows the characterization of the samples used in this study. The number-average molecular weight (Mn) and molecular weight distribution (Mw/Mn, where Mw denotes the weight-average molecular weight) were determined by gel permeation chromatography (GPC). T was measured by differential scanning calorimetry (DSC). Also, the surface glass-transition temperature (T) was deduced by the interpolation of the Mn–T relation obtained with scanning force microscopy.9
Specimens for the lap-shear test were prepared by the lamination of two PS films that had been spin-coated onto 75-μm-thick flexible polyimide (PI) films. The thickness of the PS films was controlled to be approximately 200 nm to avoid any ultrathinning effect on the thermomechanical properties.11 To remove residual solvent molecules and to eliminate stress imposed by the preparation procedure, the PS films coated on the PIs were first annealed at 393 K for at least 24 h in a vacuum oven. Then, one film was placed on a second, and the bilayer was annealed, or adhered, under a pressure of 0.35 MPa with a weight. The adhered area was fixed to be 25 mm2. After a given time, the bonding in the bilayer was quenched by immersion in liquid nitrogen.
The lap-shear test was conducted at room temperature with a Tensilon tester (RTC-1250, A&D Co., Ltd.) under the ambient atmosphere.3 Figure 1 shows the experimental setup for the measurement. A specimen prepared as previously mentioned, which was precisely cut to have a width of 10 mm and a length of 40 mm, was held at both ends with two mechanical chucks. The crosshead speed was set to be 50 μm min−1 in a tensile mode. The lap-shear strength (GL) was defined as the force corresponding to the break point divided by the adhered area of 25 mm2. To obtain reliable data, experiments with different specimens were independently repeated more than 10 times.
For the DSIMS measurement, bilayers composed of PS and dPS were prepared by a flotation method.10 The bilayers were then annealed under the same conditions used for the lap-shear test. The interfacial broadening of the bilayer by adhesion was examined (SIMS 4000, Atomika Analysetechnik GmbH). To ensure stable sputtering during the measurement, an additional buffer dPS layer was put onto the PS/dPS bilayer by the flotation technique. This approximately 100-nm-thick buffer layer removed difficulties associated with initial fluctuations in the ion beam. The incident beam of 4 keV and about 30-nA oxygen ions was focused onto a 200 μm × 200 μm area of the specimen surface. The incident angle was 45°. A thin gold layer was sputter-coated onto the bilayer surface to avoid charging of the specimen during the DSIMS measurement.
RESULTS AND DISCUSSION
Although there is no chemical bonding between two polymer layers, they can be adhered if chains are interdiffused. This is generally the case for adhesion above T. Even if the adhesion is made at a temperature below T, the polymer layers will be adhered because of interfacial broadening on a molecular level, provided that the temperature is sufficiently above T. Figure 2 shows a typical displacement–load curve obtained by the lap-shear test for a PS/PS bilayer after adhesion at 365 K for 27 h. This adhesion temperature (Tadh) falls between the values of T and T. The load monotonically increased with increasing displacement and then dropped to zero; this indicated that the PS/PS bilayer had finally failed. The GL value was obtained by the division of the force at the failure point by the contact area of 25 mm2.
Figure 3 shows a double logarithmic plot of the adhesion time (tadh) versus GL for PS (Mn = 29,000) bilayers. The GL value first increased with tadh and then reached a constant value after 105 s.13 This implies that in the initial states, the interdiffusion of chains or segments occurs even at a temperature below T, resulting in the time evolution of the interfacial adhesion strength. However, once chains or segments penetrate the depth region in which relatively large-scale molecular motion, such as segmental motion, is frozen, the interdiffusion ceases. After the time required for this to occur, about 105 s, GL becomes independent of tadh. Interestingly, the initial slope for the tadh–GL relation seems to be close to 1/4, as indicated by the broken line. This may indicate that the initial time evolution of the adhesion strength is based on the interdiffusion not of whole chains, but of segments.14
Our previous studies have revealed that the enhancement of surface mobility is dependent on Mn.9, 10 The enhancement is more pronounced for a sample with a smaller Mn value. Hence, we might expect the interfacial evolution in a bilayer composed of PS with a smaller Mn to be easier, and therefore such a bilayer would exhibit a larger GL value. On the other hand, if the interdiffusion of whole chains controls the interfacial adhesion strength, the GL value should increase with increasing Mn because of entanglements, as often seen in welding processes.1 These two factors would affect the interfacial adhesion in opposite directions. To clarify which factor is more crucial for the adhesion, GL was evaluated as a function of Mn, and Figure 4 shows the result. For this comparison, we used data from samples in which the Tadh value of 365 K was set midway between T and T,9 and tadh was set to be 5.4 × 105 s, the value at which GL reached a constant for the PS bilayers investigated.15 Although the GL value was not very sensitive to Mn, it seems that the GL value slightly decreased with increasing Mn. Hence, it is plausible that the interfacial adhesion is a diffusion-controlled process for the segments under the conditions employed. Actually, it is interesting to note that the Mn dependence of GL is in good accordance with the Mn dependence of d, which has already been published elsewhere.10
We have previously reported the time evolution of the interface for PS/dPS bilayers at a temperature below T.10 However, the samples used at that time were prepared by a procedure similar to that used here, but without the application of pressure. Hence, to make a direct comparison between GL and d, DSIMS analysis was applied to PS/dPS bilayers, which were prepared under a pressure of 0.35 MPa. Figure 5 shows typical DSIMS profiles of C−, H−, and D− ions for a PS/dPS bilayer. At first, the outermost gold layer was etched, and this made it difficult to detect secondary ions from the polymers. After the gold layer, the C− intensity started to increase and then remained almost constant through the bilayer. Thus, it can be concluded that etching proceeded at a steady state during the measurement. Although the D− intensity was stronger than the H− intensity in the buffer dPS layer, the relative intensities reversed in the PS layer. Then, the initial relation of the D− intensity to the H− intensity was recovered when the etching reached the bottom dPS layer. Postulating that a constant etching rate was attained through the bilayer, we can simply convert the abscissa of the etching time to the depth from the surface. The interfacial broadening corresponding to the interdiffusion, marked by the shaded region, was analyzed on the basis of depth profiling for the D− ion.
Assuming that the derivative of the D− intensity can be expressed by a Gaussian function, the d value was defined as twice the standard deviation, corresponding to the depth range in which the D− intensity rises from 16 to 84% of the maximum value. The broadening of the measured secondary-ion mass spectrometry profile by an instrumental function was subtracted according to the reported procedure by Whitlow and Wool.16 Figure 6 shows the time evolution of d during the adhesion process at 365 K for the bilayers of PS and dPS with an Mn value of 29,000. At first, the bilayer interface monotonically thickened with increasing tadh. When tadh exceeded 105 s, d became constant at 8.6 ± 1.2 nm. This trend quantitatively agrees with our previous results related to the time evolution of the interface for the PS/dPS bilayers,10 indicating that the applied pressure of 0.35 MPa did not affect the interdiffusion.
Takamizawa et al.17 proposed the pressure dependence of the glass-transition temperature (Tg) as follows:
where P is the pressure in excess of atmospheric, Tg,0 is the ideal Tg at P = 0, and a and c are fitting parameters. According to this equation, using reported a and c values, we could estimate the increase in Tg under the applied pressure of 0.35 MPa. The value so obtained was only 0.13 K. This is the reason that a pressure effect on the interfacial broadening was not clearly observed. The initial slope for the tadh–d relation was 1/4, as drawn by a broken line in Figure 6. This makes it clear that the initial time evolution of the bilayer interface is based on the interdiffusion of segments14 and reinforces the discussion presented for Figures 3 and 4. Also, the bilayer interface was originally composed of two film surfaces. Thus, one-half of the evolved interfacial width, 4.3 (= 8.6/2) nm, might correspond to the surface region of one film, in which the interdiffusion was attained. This value is smaller than the chain dimension of an unperturbed PS chain with an Mn value of 29,000. Therefore, our proposition of segmental interdiffusion seems to be quite reasonable.
We now come to the direct comparison of the adhesion strength with d. Figure 7 shows the relationship between d and GL for the bilayers of PS with an Mn value of 29,000, which were adhered at 365 K. The d–GL relation could be fitted by a straight line. An interesting point is that the line passes through the origin. This clearly indicates that the interfacial adhesion strength for the bilayers is controlled only by d. Because the adhesion strength is in a range up to 0.1 MPa and is much lower than the cohesive failure strength for PS, it is conceivable that the adhesive failure is taking place at the interface. Indeed, optical microscopy and atomic force microscopy observations have confirmed this point.18
There have been many attempts to improve the adhesion strength for different polymers.19–22 A common way to do so is to add an extra component such as a block copolymer, which behaves as an amphiphilic agent at the interface. Our strategy for improving the interfacial adhesion for PS layers is similar, but the extra component is added to the chain ends. We have previously studied the effect of chain ends on surface mobility in PS films23, 24 and found that the surface mobility is much enhanced for PS with hydrophobic end groups24 because the ends are preferentially segregated to the surface.12 On the basis of this notion, we tried to improve the interfacial adhesion. Figure 8 shows the time evolution of GL for α,ω-PS(Rf)2 layers adhered at 365 K. For comparison, the data set for the conventional PS bilayers is also plotted in the figure. The GL value increased with increasing tadh, with a slope of 1/4 for both PS and α,ω-PS(Rf)2. However, the GL value was always higher for α,ω-PS(Rf)2 than for PS at a given time. α,ω-PS(Rf)2 has fluoroalkyl chain ends, which are lower surface energy components compared with the main-chain part. Hence, preferential surface segregation of the chain ends was expected and was confirmed by X-ray photoelectron spectroscopy.12 Also, T for α,ω-PS(Rf)2 was lower than that for PS.24 As a result, the difference between the Tadh value of 365 K and the T value was larger for α,ω-PS(Rf)2 than for PS, and this resulted in a thicker interface for α,ω-PS(Rf)2. Thus, it can be envisaged that the adhesion strength can be regulated under given conditions by changes in the chain-end chemistry.
Figure 9 shows the d dependence of GL for the α,ω-PS(Rf)2 and PS layers. At a given time, the GL value for α,ω-PS(Rf)2 was higher than that for PS, as shown in Figure 8. However, the d–GL relation for α,ω-PS(Rf)2 and PS can be superimposed into one line, which passes through the origin. This again makes it clear that the adhesion strength is controlled only by d and adds further weight to the conclusions concerning the chain-end effect drawn from Figure 8.
Interfacial adhesion promoted by enhanced surface mobility was studied for symmetric PS layers. During the initial stages of adhesion for the PS layers at a temperature below T, the interfacial adhesion strength and width increased with increasing time on account of the interdiffusion of segments. The adhesion strength was controlled by the interfacial width. Finally, it was demonstrated that changing the chain-end chemistry could improve the adhesion strength.
This research was partly supported by Industrial Technology Research Grant Program in 2006 from the New Energy and Industrial Technology Development Organization of Japan and by Grants-in-Aid for Young Scientists (A 18685014) and for the 21st Century COE Program “Functional Innovation of Molecular Informatics” from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.