Sum Frequency Generation Studies at Poly(ethylene terephthalate)/Silane Interfaces:  Hydrogen Bond Formation and Molecular Conformation Determination

Abstract

To better understand the effects of interfacial molecular orientation on adhesion to plastics, the interfaces between poly(ethylene terephthalate) (PET) and different silane coupling agents were probed using sum frequency generation (SFG) vibrational spectroscopy. The polymer/air interface was dominated by the ester carbonyl, methylene, and phenyl groups. Upon contacting the PET film with the amino-functional silane 3-aminopropyltrimethoxysilane (ATMS), the ester carbonyl stretch shifted to a lower energy indicating the formation of hydrogen bonds between the polymer surface and the silane molecules. This shift was not observed when silanes that contained no hydrogen bond donors, such as (3-glycidoxypropyl)-trimethoxysilane and n-butyltrimethoxysilane, were placed into contact with the PET surface. Further evidence of silane ordering at the interface was observed as vibrational peaks attributed to the C-H stretching of the silane methoxy headgroups dominated the PET/silane spectra. It was determined that the conformation of the ATMS molecules at the interface was such that the amino endgroups were oriented toward the interface while the methoxy headgroups were directed toward the silane bulk.

Diffusion of one or more components of a silane adhesion-promoting mixture into poly(methyl methacrylate)

“…In previous reports, no evidence of diffusion of γ -GPS or the SAPM into PET was observed [10,13] and ATMS and AATMS were found to diffuse into PMMA and PS [11]. Here the SFG data indicate that γ -GPS dissolves PS and diffuses into PMMA.…”

“…This is similar to the polymer/ATMS interfaces. Here, ATMS diffused into PMMA [11] but rather hydrogen bonded to the surface ester carbonyl groups of PET and formed a stable interface [10].…”

“…The diameter of both beams is about 0.5 mm. Previous research using this experimental setup shows that the SFG signals are dominated by the polymer/air or polymer/liquid interfaces with negligible contributions from the polymer/substrate interface or the polymer bulk [10,14,15]. All SFG spectra were collected using the ssp (s-polarized sum frequency output, s-polarized visible input, and p-polarized IR input) polarization combination and were normalized by the intensities of the input visible and IR beams.…”

“…Unfortunately, the molecular-level mechanisms of the adhesion enhancement provided by this SAPM at polymer/silicone elastomer interfaces are not yet fully understood because of the inability of most surface analytical methods to probe buried interfaces in situ. Recently, sum frequency generation (SFG) vibrational spectroscopy has been applied to the study of polymer/silane interfaces [9][10][11][12][13]. SFG is a nonlinear optical laser technique that is inherently surface-and interface-sensitive and has proven to be an invaluable tool to examine polymer surfaces and interfaces [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26].…”

“…Recently, sum frequency generation (SFG) vibrational spectroscopy has been applied to the study of polymer/silane interfaces [9][10][11][12][13]. SFG is a nonlinear optical laser technique that is inherently surface-and interface-sensitive and has proven to be an invaluable tool to examine polymer surfaces and interfaces [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. Our SFG studies at the interfaces between polymers (e.g., poly(methyl methacrylate) (PMMA), polystyrene (PS), and poly(ethylene terephthalate) (PET)) and organo-functional silanes have elucidated the conformations of interfacial silane molecules [9,10], monitored interfacial diffusion of silanes into a polymer film [11], and detected interfacial hydrogen bonding [10].…”