In the field of infrared spectroscopy characterization of polymer films, functional coatings, and flexible electronic materials, traditional transmission methods have long been regarded as the preferred solution for obtaining high-signal intensity spectra. However, with the widespread application of new materials such as multilayer composite films, high-transmittance ultrathin films, and opaque functional films, the inherent limitations of transmission methods have gradually become apparent.
Attenuated Total Reflection (ATR), with its advantages such as surface selectivity, non-destructive testing, and minimal sample pretreatment, has gradually become increasingly widely used in characterization and inspection of various thin film samples. This paper reviews the principles and application differences between ATR and transmission methods, and provides standardized flexible material auxiliary enhancement solutions to address common bonding challenges in film samples.
1. Comparative Analysis of the Principles of ATR and Transmission Methods
In Fourier transform infrared spectroscopy, the ATR method and the traditional projection method are two common mainstream detection methods, but they differ significantly in principle and application compatibility. When measuring plastic film samples, the main technical differences between the two are shown in Table 1 below, and the spectral comparison is shown in Figure 1
|
Item |
Transmission method |
ATR method |
|
Optical signals |
Sample is fully stacked for absorption |
Selective absorption at the micron level on the surface |
|
Sample thickness |
Films made of highly absorbent materials Signal saturation issues are likely to occur beyond 20-30μm |
There are usually no strict thickness limits Mm-level films can be directly inspected |
|
Opaque samples |
Completely untestable |
It can be tested directly |
|
Film performance |
Spectra are easily saturated, and characteristic peaks are distorted |
Normally collects effective signals with no risk of saturation |
|
Sample pretreatment |
Needs to be sliced and fixed to the KBr window plate |
Ensure the test surface is flat and requires no additional processing |
Figure 1 Comparison of spectra for plastic film determination by transmission and ATR methods
2 Pain points in ATR testing for thin film samples and auxiliary enhancement solutions for flexible materials
The signal strength of the ATR method highly depends on the close adhesion of the optical stage between the sample and the crystal surface. For ultrathin films with thicknesses less than 20μm or micron-scale concave-convex deformation, even when applying rated pressure with standard metal indenters, local microgaps still occur, preventing effective coupling of the lost waves into the sample. Ultimately, this results in insufficient characteristic peak strength, complete loss of detail peaks of low-concentration functional groups, and a significant decrease in signal-to-noise ratio
To address this challenge, using flexible material-assisted lamination technology can achieve signal enhancement without additional hardware modifications. The operation is as follows:
Types of auxiliary materials: Priority is given to methyl vinyl silicone rubber (silicone sheets) with a Shore hardness of 30~40 HA, or natural rubber, modified soft polyolefin sheets, etc
Material thickness range: The thickness of auxiliary flexible sheets is strictly controlled within the 0.5~1.0mm range. If the thickness is less than 0.3mm, there is insufficient deformation buffer and cannot fill the micro-uneven structures on the film surface; if the thickness exceeds 1.5mm, the indenter stress may be excessively dispersed, which actually reduces the actual contact pressure between the sample and the crystal
Figure 2: Flexible material without padding (left) VS flexible material with padding (right)
Deformation control indicators: During lamination, the deformation of the flexible material is controlled within the range of 15%~30%. The indenter pressure of the diamond ATR crystal is set to 0.6~0.8MPa. Under this parameters, the flexible material can fully replicate the microstructure of the film's backside, transmitting uniform surface pressure across the entire film region, significantly improving the contact and adhesion rate between the film and the crystal surface. Measured uneven rough film signal values can increase by about 40%.
3 Typical cases of ATR testing for special composite films
A composite film produced by a certain research institute was selected as the test subject. The total thickness of the sample was about 80μm, with a rough visual surface containing fine needle-like substances
Direct testing using transmission method: multiple spectral range signals are saturated, making it impossible to determine specific information;
Direct lamination testing using conventional ATR method: Due to fine indentations on the thin film surface and insufficient adhesion, the characteristic peak strength of C=C (1590 cm⁻¹) is only 0.063 Abs, and that of C-O (1300 cm⁻¹) is only 0.032 Abs, indicating poor qualitative determination;
Optimized testing using the above flexible silicone pad-assisted method: the characteristic peak strength of C=C(1590 cm⁻¹) increases to 0.394 Abs, the signal is improved sixfold, and C-O (1300 cm⁻). ¹) The characteristic peak intensity was increased to 0.152 Abs, with the signal increasing nearly fivefold. At the same time, a complete characteristic vibration peak was captured at 890 cm⁻¹, which typically represents the C-H bending vibration of the aromatic hydrocarbon mono-substitution or adjacent substitution. This information corresponds with the C=C stretching vibration of the aromatic ring (1590 cm⁻¹) and the C-H bending vibration of the parasymmetrical substitution of the aromatic ring (810 cm⁻¹), confirming the presence of aromatic rings in the film. Additionally, the number of effective peak recognition in the overall spectrum has increased from 13 to 19, greatly enriching the qualitative information on the functional groups on the thin film surface. See the figure and table below for details
Figure 3 Comparison of the spectrum of flexible materials with pads (red) VS flexible materials without pads (green).
| Infrared test peak table—comparison of samples with pad-added flexible materials and those without padding | |||
| NO. | Peak positioncm-1 | Absorbance Abs | |
| Flexible materials are added to the pad | Flexible materials are not added | ||
| 1 | 625 | 0.389 | —— |
| 2 | 656 | 0.386 | 0.138 |
| 3 | 669 | 0.395 | 0.154 |
| 4 | 718 | 0.282 | —— |
| 5 | 779 | 0.22 | —— |
| 6 | 814 | 0.255 | 0.078 |
| 7 | 887 | 0.158 | —— |
| 8 | 947 | 0.188 | 0.046 |
| 9 | 1026 | 0.614 | 0.103 |
| 10 | 1082 | 0.307 | 0.048 |
| 11 | 1124 | 0.192 | 0.04 |
| 12 | 1142 | 0.160 | 0.035 |
| 13 | 1173 | 0.117 | —— |
| 14 | 1208 | 0.066 | —— |
| 15 | 1302 | 0.152 | 0.032 |
| 16 | 1408 | 0.302 | 0.052 |
| 17 | 1591 | 0.394 | 0.063 |
| 18 | 2925 | 0.058 | 0.004 |
| 19 | 3242 | 0.145 | 0.012 |
At the same time, this testing solution does not require complex preprocessing such as resin encapsulation or microslicing of samples; the total time for a single sample test is only about one minute, fully meeting the rapid quality inspection needs of industrial production lines.
4 Conclusion
With its core advantages of surface selectivity, non-destructiveness, and easy pre-treatment, the ATR method has become a better alternative for infrared characterization of plastic film samples in scenarios where transmission methods are not suitable.
The standardized flexible material-assisted enhancement technology proposed in this paper can significantly improve ATR test signal strength, characterization peak reliability, and data stability levels of ultra-thin films without increasing hardware costs or technical upgrades. This method has been validated in typical application scenarios in polymer packaging and flexible electronic coatings, providing a more precise and stable solution for plastic film sample detection using Fourier transform infrared spectrometers.
Post time: Aug-28-2026