Article pubs.acs.org/JPCB
Crystallization of Polymer Chains Chemically Attached on a Surface: Lamellar Orientation from Flat-on to Edge-on Yihuang Chen, Tiansheng Gan, Chunfeng Ma, Linge Wang, and Guangzhao Zhang* Faculty of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, P. R. China S Supporting Information *
ABSTRACT: Crystallization of polymer chains confined on a surface greatly influences surface properties. We have grafted comb-like copolymer, consisting of poly(2-hydroxyethyl methacrylate) (PHEMA) backbone and semicrystalline poly(εcaprolactone) (PCL) side chains, on silicon surface and investigated the crystallization of such confined PCL chains upon solvent evaporation by using atomic force microscopy (AFM), grazing incidence wide-angle X-ray scattering (GIWAXS), and polarized optical microscope (POM). Our studies reveal that the PCL chains align and form “flat-on” lamellae at a low PCL chain density. As the chain density increases, the comblike polymer (PHEMA-g-PCL) chains undergo pancake-to-mushroom-to-brush transition, and the lamellae turn from “flat-on” to “edge-on” in orientation. Further increasing PCL chain density leads the “edge-on” lamellae to change from a quasi-twodimensional (quasi-2D) to quasi-three-dimensional (quasi-3D). Because of the confinements of polymer chains, we can observe the evolution of spherulites at different stages during the mushroom-to-brush transition of PHEMA-g-PCL chains. The confinements also result in knobbly substructures in the edge-on lamellae.
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INTRODUCTION Polymer crystallization which deals with the degree of crystallinity and the size and orientation of the polymer chains, is an old issue but has received sustainable interest because it dramatically affects the optical, mechanical, thermal, electrical and chemical properties of polymers.1,2 Polymers can crystallize from a melt, solution, solvent evaporation or mechanical stretching. When polymers crystallize from an isotropic melt or concentrated solution, the chains fold into 10 to 20 nm thick lamellae, which further organize into spherulites.3,4 When polymer chains are confined on a surface, the isotropic spherulitic organization of lamellar crystals is restricted, leading to unique lamellar crystal orientations, where the chain alignment can be parallel or perpendicular to the surface forming the so-called “edge-on” and “flat-on” lamellar orientation, respectively.5−7 In a film with thicknesses of 100−1000 nm, polymer chains generally form edge-on lamellae.8−12 In a much thinner film with thicknesses less than 100 nm, they usually form flat-on lamellae,13−17 but edgeon lamellae were also reported.18,19 The thickness effect on lamellar orientation was explained in terms of thermodynamics. In ultrathin film, the polymer chains favor alignment perpendicular to the surface with flat-on lamellar orientation to minimize to surface energy during the primary nucleation in crystallization. In a thicker film, the polymer chains parallel to the surface can effectively decrease the surface energy for the primary nucleation, leading to edge-on lamellar orientation.10 The lamellar orientation is also determined by the nature of polymer chains and the crystallization conditions. Polylactide © XXXX American Chemical Society
on a surface isothermally crystallized from a melt was reported to form lamellar crystal with orientation from edge-on to flat-on depending on the crystallization time.20,21 Polystyrene-bpoly(ethylene oxide) chains in ultrathin film form flat-on lamellae in the early stage when they are annealed in toluene vapor. As the solvent molecules gradually diffuse into the polymer/substrate interface, edge-on lamellae result.22 Poly(ethylene oxide) film prepared by spin-coating preferentially has edge-on lamellar orientation at a thickness above 1000 nm, but it favors flat-on orientation at a thickness less than 300 nm.23,24 Poly(ε-caprolactone) (PCL) film with thickness of 200−300 nm mainly exhibits flat-on lamellae, where PCL chains are physically adsorbed on the surface by solventcasting.25 When polymer chains are chemically grafted on a surface and form brushes, their crystallization are more complex because they have stretching conformation and are confined in a space consisting of the surface and the surrounding chains. It is reported that poly(methoxypoly(ethylene glycol) methacrylate) (PMPEGMA) brushes form flat-on and edge-on lamellae at small and larger thickness, respectively.26 However, only compact flat-on lamellae was observed for PCL grafted on a surface.27−29 So far, the mechanism about the formation of flaton and edge-on lamellae of polymer chains confined on a surface remains unclear. Received: March 5, 2016 Revised: May 1, 2016
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DOI: 10.1021/acs.jpcb.6b02344 J. Phys. Chem. B XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry B
for 12 h. The surface was then washed with ethanol and Milli-Q water, blown with nitrogen. The amino-modified silicon wafer was placed in a solution of 0.8 mL of TEA in 30.0 mL of THF under nitrogen atmosphere. After BIBB (0.62 mL) in 20.0 mL of THF was added dropwise into the mixture at 0 °C, it was stirred for 12 h at 25 °C. The wafer was washed with THF, Milli-Q water, methanol and acetone, blown dry with nitrogen, yielding an initiator-modified silicon wafer. PHEMA chains were grafted via SI-ATRP. HEMA (4.0 mL, 32.97 mmol), CuCl2 (21.6 mg, 0.16 mmol), and bpy (244.0 mg, 1.56 mmol) were added to 4.0 mL of water in a Schlenk tube capped with a rubber septum and equipped with a magnetic stir bar. The mixture was stirred until a homogeneous blue solution formed and degassed by three freeze−pump− thaw cycles. Then, CuCl (55.0 mg, 0.55 mmol) was introduced, and the solution was stirred until it turned into dark brown. The initiator-modified silicon wafer was placed inside the Schlenk tube under the protection of nitrogen at 25 °C. After a certain time, the sample was removed from solution, ultrasonically cleaned in ethanol, Milli-Q water, and THF, and then dried under a nitrogen flow. PCL chains were then grafted on PHEMA backbone via ROP of CL monomer with the hydroxyl groups as the initiating sites and t-BuP4 as the catalyst.36 Since such polymerization is living, the molecular weight of PCL and layer thickness were tuned by ε-caprolactone concentration. Typically, PHEMAmodified wafer, CL (1.15 g, 10 mmol) and THF (4.0 mL) were placed in a flamed and nitrogen purged Schlenk tube. The mixture was degassed by three freeze−pump−thaw cycles. tBuP4 (50.0 μL, 0.05 mmol) was added through a rubber septum with a syringe to start the polymerization at 25 °C. The sample was removed from solution after 24 h, ultrasonically cleaned in THF to remove possible impurities and physically adsorbed polymers, and then dried under a nitrogen flow at 25 °C. The crystallization of PCL chains took place during solvent evaporation. For comparison, the PCL crystallization under quenching or annealing conditions was also examined. The sample was quenched in liquid nitrogen after the brushes together with the silicon wafer was heated to 120 °C (above the Tm of PCL) for 40 min under nitrogen atmosphere to remove the thermal history. On the other hand, the melted sample cooled down at 45 °C for 7 h under nitrogen atmosphere to yield the annealed sample. The sample was observed by AFM immediately after quenching or annealing. POM Observation. The morphology of the polymer layers was observed on an Axioskop 40-POL polarized optical microscope (POM) (Zeiss, Germany) with a charge-coupled device (CCD) record system. AFM Observation. The topology of the polymer layers was observed by using AFM (XE-100, Park Systems) in air. A NCHR cantilever having a nominal spring constant of 42 N/m was used with etched silicon tips having a nominal radius of curvature of 10 nm. Topography and phase images were recorded simultaneously in noncontact mode at a scan rate of 0.5 Hz at 25 °C. The root-mean-square (RMS) surface roughness was estimated from the roughness profiles. Generally, the RMS roughness was taken over an area of 10 μm × 10 μm. For annealed samples, since the spherulite was larger than 10 μm × 10 μm, it was obtained over an area of 30 μm × 30 μm. The measurement of the polymer layer thickness was based on AFM profilometry. After the polymer chains were partially removed by scratching with sharp razor blades (width of scratch between 5 and 30 μm), the thickness was taken by
In the present work, we have grafted comb-like copolymer consisting of poly(2-hydroxyethyl methacrylate) (PHEMA) backbone and PCL side chains onto a silicon surface via surface-initiated atom transfer radical polymerization (SIATRP) and ring-opening polymerization (ROP) (Figure 1),
Figure 1. Schematic illustration of grafting of PHEMA-g-PCL chains on silicon surface.
and investigated the crystallization of the confined PCL chains upon solvent evaporation by use of polarized optical microscope (POM), atomic force microscopy (AFM) and grazing incidence wide-angle X-ray scattering (GI-WAXS). Owing to the sufficient hydroxyls in PHEMA chain, the PCL chain density can be regulated by ε-caprolactone concentration in the preparation. We have examined the lamellar orientation and the crystal morphology. Because of the conformation, PCL form well-round spherulites with featured Maltese cross patterns. Our aim is to understand the crystallization mechanism of confined polymer chains. As a biodegradable polymer, PCL has found applications in biomedical engineering30,31 and marine antibiofouling.32,33 Our other aim is to provide useful information for development of PCL based surfaces and interfaces.
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EXPERIMENTAL SECTION Materials. Silicon wafer (n-doped, (100) orientation, 0.525 mm in thickness and 100 mm in diameter) purchased from the Guangzhou Institute of Semiconductor Materials was cut into square chips of ∼0.5 cm × 0.5 cm. 3-Aminopropyltriethoxysilane (APTES, 99%) and CuCl2 (98%) from Aladdin were used as received. CuCl (AR) from Aladdin was stirred in acetic acid, washed with ethanol and dried in vacuum. ε-Caprolactone (CL) from Aldrich was dried over calcium hydride (CaH2) and distilled under reduced pressure before use. Triethylamine (TEA) and tetrahydrofuran (THF) from Sinopharm were distilled over CaH2. 2-Hydroxyethyl methacrylate (HEMA) from Aldrich was distilled over copper powder under reduced pressure. Toluene and methanol from Sinopharm were refluxed and distilled in the presence of sodium. 2-Bromoisobutyryl bromide (BIBB, 98%), 2,2′-dipyridyl (bpy, 99%) and (1-tertbutyl-4,4,4-tris(dimethylamino)-2,2-bis[tris(dimethylamino)phosphoranylidenamino]-2Λ5,4Λ5-catenadi(phosphazene) (tBuP4, ∼ 0.8 M in hexane) from Aldrich were used without further purification. Other reagents from Sinopharm were used as received. Grafting of PHEMA-g-PCL Chains on Silicon Surface. The details about preparation of the initiator-modified silicon wafer can be found elsewhere.34,35 To obtain a hydroxyterminated silicon surface (Si−OH surface), the cleaned silicon wafer was immersed in a freshly prepared piranha solution consisting of a mixture of H2SO4 and H2O2 (7:3 v/v) at 100 °C for 60 min, rinsed with Milli-Q water, and dried under a nitrogen flow. The amino-modified silicon wafer was obtained by immersing the above Si−OH surface in a solution of 0.6 mL of APTES in 30.0 mL of toluene at 25 °C for 12 h and 80 °C B
DOI: 10.1021/acs.jpcb.6b02344 J. Phys. Chem. B XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry B
PCL crystalline morphology as a function of PHEMA-g-PCL layer thickness was observed by AFM. Figure 3 shows that the
AFM as the distance between the bare silicon surface and the average height in a line scan across the top of the remaining polymer layers.37 For each sample, we used the thickness from averaging those of five measurements at different locations. GI-WAXS Measurements. Grazing incidence wide-angle X-ray scattering (GI-WAXS) measurements were carried out on a Xenocs Xeuss 2.0 SAXS/SAXS system equipped with a 2D area detector (2D Pilatus 100k) and a Cu K radiation source (λ = 1.54 Å). The sample to detector distance was 127 mm. The 2D patterns were subjected to incident beam intensity and background corrections. Some scattering experiments were also undertaken on a PANalytical X’Pert PRO X-ray diffractometer equipped with a Cu Kα radiation source (λ = 1.54 Å). The diffracted beam is in the plane defined by the incidence beam and the surface normal. This geometry is sensitive to the structure parallel to the surface.38 Grazing incidence angle of 0.5° was tested. The data were scanned in the 2θ range of 20− 25°.
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RESULTS AND DISCUSSION First, we examined the crystallization of the grafted PCL chains as a function of PHEMA-g-PCL layer thickness by using POM (Figure 2).
Figure 3. AFM topography images of PHEMA-g-PCL layer with different thicknesses: (a) 72 nm; (b) 89 nm; (c) 110 nm; (d) 179 nm; (e) 204 nm; (f) 221 nm. The insets show the zoom-in images (a−f, 1 μm × 1 μm). Arrows indicate the flat-on lamellae.
layers with and without grafting PCL chains exhibit quite different morphologies. The latter (RMS roughness
