Annealing Effect on Surface Crystal Structure of Polyethylene Thin Films Fig. 1. Schematic geometry of the in-plane GIXD measurement. α i : incident angle, α f : take-off angle, q xy : scattering vector. The molecular aggregation structure at the surface of polymeric materials is strongly related to their physical properties such as friction and adhesion. However, the intrinsic features of the molecular aggregation structure at the surface of crystalline polymers have not yet been clarified on the molecular scale. We have been investigating the crystal structure at the solid surface of polyethylene (PE), a typical example of crystalline polymers, by grazing incidence X-ray diffraction (GIXD) measurements. In the case of melt-crystallized PE thin films, it was revealed that the lattice dimension in the in-plane direction was smaller in the near-surface region than in the bulk region. Moreover, we found for the first time that the paracrystalline lattice distortion in the in- plane direction was larger in the near-surface region than in the bulk region but the crystallite size was almost the same between these two regions [1]. On the other hand, Burell and Wehrum et al. reported the local alignment of polymer chains at the surface of spin-cast poly(ethylene telepthalate) thin films during annealing [ 2 ]. In order to control the surface structural and physical properties of polymeric materials, it is important to clarify the chain packing structure at the surface of crystalline polymers under various conditions. In this study, the annealing effect on crystal structure at the surface was investigated for PE thin films by synchrotron-source GIXD measurements. High-density polyethylene without additives (HDPE ; Hizex 1 300J , MI = 1 4 , Mitsui C hemicals, Inc.) was used as a sample. Thin films with a thickness of ca. 400 nm were prepared onto silicon substrates from a p -xylene solution of HDPE by a dip-coating method. Thus, the obtained films were annealed at various temperatures ( T a ) for 24 hours under nitrogen atmosphere. F igure 1 shows the schematic geometry of the in-plane GIXD measurement. In order to measure reflections from crystallographic planes perpendicular to the sample surface, in-plane GIXD measurements were carried out for the films at beamline BL13XU [ 3 ] using incident X-rays with a wavelength of 0 .1 280 nm. When the incident angle ( α i ) of X-rays to the sample surface is smaller than the critical angle ( α c ), the X- rays undergo total external reflection and penetrate into the sample as evanescent waves. A round α c , the penetration depth changes drastically from several micrometers to several nanometers. Therefore, surface-sensitive and bulk-sensitive profiles were obtained by in-plane GIXD measurements at α i = 0 .11 ( < α c ) and 0 . 20 ( > α c ), respectively. F igure 2 shows the annealing temperature dependence of the lattice constants a , b and c of the orthorhombic unit cell in the near-surface and bulk regions of HDPE thin films calculated on the basis of in-plane GIXD profiles. The a and b in the near- surface region of the as-prepared films were smaller than those in the bulk region. This tendency was also observed for the films annealed at high T a which would be released from residual stress induced by dip-coating. Therefore, in-plane GIXD analysis implied that the crystal density in the near-surface region would be essentially higher than that in the bulk region. In the near-surface region, PE chains were considered to be packed closely together to minimize the surface free energy of the films. Paracrystalline lattice distortion in the in-plane direction was evaluated with the (11 0 ) reflection and its higher- order reflections on the basis of the p a r a c r y s t a l l i n e l a t t i c e a n a l y s i s proposed by Hosemann [ 4 ]. In the paracrystalline lattice model, the lattice vectors of ad j acent unit cells vary in magnitude and direction due to the large displacement of lattice points from their ideal positions, which results in a loss of the long-range crystallographic order. The integral width δβ of a 63 References [ 1] H. Yakabe, S. Sasaki, O. Sakata, A. Takahara and T. Kajiyama: Macromolecules 36 (2003) 5905. [2] M. Durell e t al. : E uro p hy s. L ett. 58 ( 2002) 844 . [ 3] O. Sakata, e t al. : Sur f . R e v . L ett . 10 (2003) 5 4 3. [ 4 ] P .H . L i ndenmeyer and R . Hosemann: J . A pp l. P hy s. 34 (19 6 3) 4 2. Fig. 2. Annealing temperature dependence of the lattice constants a, b a b nd c of the orthorhombic unit cell in the near-surface and the bulk regions measured for dip-coated HDPE thin films. Open circles: near-surface region, filled circles : bulk region. So no Sasaki a, * , Atsushi Takahara b and Tisato Kajiyama c a a (a) G raduate School o f E n g ineerin g , Kyushu U ni v ersity (b) I nstitute f or Materials C hemistry and E n g ineerin g , Kyushu U ni v ersity (c) Kyushu U ni v ersity * Present address : S P rin g-8 s / J AS RI E- mail: sono @ s p rin g 8 .or.j p Fig. 3. Annealing temperature dependence of g (110) (a) and N (110) (b) for dip-coated HDPE thin films. Open circles: near-surface region, filled circles: bulk region. 340 350 360 370 380 390 400 0.250 0.253 0.254 0.256 0.258 0.260 c ( nm) Annealing Temperature (K) a (nm) 0.7 41 0.742 0.743 0.745 0.746 0.747 0.744 0.740 0.493 0.494 0.495 0.497 0.498 0.499 0.496 0.492 bu lk near-surface b (nm) 3 40 350 360 370 380 390 400 near-surface bulk Annealing Temperature (K) 30 35 40 45 50 55 60 65 N (110) N N ( b) g (110) ( × × 10 -2 ) 3 40 350 360 370 380 390 400 Annealing Temperature (K) bulk near-surface 3. 0 1.0 2.0 4.0 5.0 ( a) re f lectio n is e xp ressed by ( δβ ) β β 2 = ( 1 / ã 2 )[(1 / N 2 ) + π 4 g 4 h 4 ]. Here, g is the standard de v iatio n o f th e G aussia n distribution di v ided by t he a v era g e lattice v ecto r ã , h is the scatterin g order and N is the number o f scatterin g u nits. I n other w ords, g is a p arameter used to e v aluate the de g ree o f p aracrystalline disorder and N corres p onds to crystal si z e . F i g ures 3(a) and 3(b) sho w the annealin g tem p erature de p endences o f g (110) and N (110) N N in the in -p l ane [110] direction in the near - su r f ace a nd bulk re g ions o f the HD PE thi n f il ms, res p ecti v ely. I n the case o f the as -p re p ared f il ms, the (220) re f lectio n f rom the near - su r f ace r e g ion w as not clearly detected. This indicated e x tremely lar g e lattice distortion. Also, the g (110) in the near - sur f ace re g ion o f the annealed f ilms w as l ar g er than that in the bulk re g ion. There f ore, p aracrystalline lattice distortion in the in -p l ane direction w ould be essentially lar g er in the near - su r f ace r e g ion than in the bulk re g ion. On the other hand, the N (110) N N in the near - su r f ace r e g ion w as smaller than or the same as that in the bulk re g ion. The decrease in g (110) and the increase in N (110) f rom T a T = 3 4 3 K to 393 K w ere l ar g e in the near - su r f ace r e g ion. These results su gg est that structural reor g ani z ation accom p anyin g p artial - meltin g and recrystalli z ation drastically occurred in the near - sur f ace re g ion o f the f il ms. 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 6 6 6 6 6 6 6 64 64 64 64 64 64 64 64 64 64 64 64 64 64 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
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