Retention of poly(N-isopropylacrylamide) on 3-aminopropyltriethoxysilane

Abstract

Silane coupling agents are commonly employed to link an organic polymer to an inorganic substrate. One of the widely utilized coupling agents is 3-aminopropyltriethoxy silane (APTES). In this study, the authors investigated the ability of APTES to retain thermo-responsive poly(N-isopropylacrylamide) (pNIPAAm) on hydroxylated surfaces such as glass. For comparison purposes, the authors also evaluated the retention behaviors of (1) polystyrene, which likely has weaker van der Waals interactions and acid-base interactions (contributed by hydrogen-bonding) with APTES, on APTES as well as (2) pNIPAAm on two other silane coupling agents, which have similar structures to APTES, but exhibit less interaction with pNIPAAm. Under our processing conditions, the stronger interactions, particularly hydrogen bonding, between pNIPAAm and APTES were found to contribute substantially to the retention of pNIPAAm on the APTES modified surface, especially on the cured APTES layer when the interpenetration was minimal or nonexistent. On the noncured APTES layer, the formation of an APTES-pNIPAAm interpenetrating network resulted in the retention of thicker pNIPAAm films. As demonstrated by water contact angles [i.e., 7°-15° higher at 40 °C, the temperature above the lower critical solution temperature (LCST) of 32 °C for pNIPAAm, as compared to those at 25 °C] and cell attachment and detachment behaviors (i.e., attached/spread at 37 °C, above LCST; detached at 20 °C, below LCST), the retained pNIPAAm layer (6-15 nm), on both noncured and cured APTES, exhibited thermo-responsive behavior. The results in this study illustrate the simplicity of using the coupling/adhesion promoting ability of APTES to retain pNIPAAm films on hydroxylated substrates, which exhibit faster cell sheet detachment (≤30 min) as compared to pNIPAAm brushes (in hours) prepared using tedious and costly grafting approaches. The use of adhesion promoters to retain pNIPAAm provides an affordable alternative to current thermo-responsive supports for cell sheet engineering and stem cell therapy applications.

Fig. 1.

Thickness of (a) APTES layers cured for different durations, (b) the retained pNIPAAm and (c) the retained PS, which had been thermally annealed for 3 days on these APTES layers. The rinsing and soaking were done using cold DI water for pNIPAAm and toluene for PS. The red and green dashed lines are used to indicate, respectively, the thickness of APTES alone after rinsing and after soaking. The silica layer thickness (1.5–2 nm) was subtracted and is not included in the thickness values.

Fig. 2.

Water contact angles of APTES layers (prepared by spin-coating 1 wt. % APTES in ethanol) cured with different curing times at 160 °C inside a vacuum oven (a) before and (b) after rinsing + 3 days of soaking of the APTES layers in cold DI water.

Fig. 3.

(a) Retained pNIPAAm layer (after rinsing with and soaking in cold DI water, measured in air) after pNIPAAm films had been thermally annealed on SiO_x and various noncured silanes for 3 days at 160 °C. The variation of the retained film thickness on MPTMS and TESBA was greater as compared to that of APTES, indicating that MPTMS and TESBA could be more sensitive to the preparation conditions. (b) The thickness of one set of pNIPAAm on the above three silanes measured in water at a temperature above (40 °C) and below (25 °C) the LCST ( ∼32 °C) of pNIPAAm along with their values in air. The films in water were slightly thicker (1.5–2x) than those in air. No thickness difference in water at 25 and 40 °C was observed for pNIPAAm retained on MPTMS and TESBA.

Fig. 4.

Potential mechanisms of retaining polymers on APTES: (a) IPN formed during thermal annealing + van der Waals (vdW) + AB interactions for pNIPAAm on the noncured APTES layer and (b) vdW + AB interactions for the polymer on the cured APTES layer. (c) and (d) represent the XPS spectra that illustrate the retention of pNIPAAm and PS, respectively. c-1 and c-2 are for the pNIPAAm film cured on noncured APTES for 3 days at 160 °C before and after soaking (3 days) in cold water, respectively; c-3 and c-4 are for the pNIPAAm film cured on 2 h-cured APTES and on SiO_x, respectively, for 3 days at 160 °C and then rinsed with cold water; d-1 and d-2 are the PS film cured on noncured APTES for 3 days at 125 °C before and after rinsing with toluene, respectively; d-3 and d-4 are for the PS film cured on 2 h-cured APTES and SiO_x, respectively, for 3 days at 125 °C and then rinsed with toluene.

Fig. 5.

Decoupling of C1s peaks of samples c-1, c-2, c-3, c-4, d-1, d-2, and d-3 presented in Fig. 4. The C1s peaks for c-1, c-2, c-3, and c-4 are similar, and they can be decoupled into three main peaks at binding energies of ∼285 eV (C–C/C–H), 286 eV (C–N), and 288.5 eV (N–C=O). For d-1 (∼30 nm PS film on APTES), clear π-π* shake-up satellites were observed at ∼291.5 eV, and this peak was barely noticed for d-2 and d-3. Also, d-1 has a symmetric peak, it could be forcedly decoupled into two peaks (aromatic C–C and aliphatic C–C), but one peak (C–C/C–H) showed a better fit. For d-2 and d-3, a small peak associated with the C–N bond was observed, and for d-3, another small peak at ∼288 eV was noticed. For d-4, broadening of the peak at high BE was noticed. The relevant numeric values of peak area % are summarized in Table II.

Fig. 6.

Optical microscopy images of a sheet of mouse embryonic fibroblast (NIH3T3) cells detached from the pNIPAAm film retained on (a) a noncured and (b) a 3 day cured APTES layer. The time indicated in each image was the elapsed time of the culture dish being removed from the 37 °C incubator and placed on the microscope stage in the ambient condition (∼23 °C, 1 atm). The scale bar is 500 μm.