Results and Discussion
Semi Crystalline Blends
In the last decade there has been a substantial and growing interest in polymer "thin" films. These films (typically below 200 nm thickness) show remarkable properties, sometimes completely different from those in bulk. Understanding how the morphology of thin polymer (blend) films evolves with time or depends on preparation methods is of great technological importance. AFM turns out to be the suited technique for studying these films since it makes it possible to obtain different images (e.g. topographic, friction and phase) in one scan. The nano-TA thermal probe from Anasys Instruments adds a new and valuable capability of spatially resolved thermal analysis to the AFM. It is particularly useful for thin films since it enables the measurement of transition temperatures (melting or glass) on selected spots of the sample aiding in the identification and characterization of the phases.
Nano-TA thermal probe is a local thermal analysis technique which combines the high spatial resolution imaging capabilities of atomic force microscopy with the ability to obtain understanding of the thermal behaviour of materials with a spatial resolution of sub-100nm. (a breakthrough in spatial resolution ~50x better than the state of the art, with profound implications for the fields of Polymers and Pharmaceuticals). The conventional AFM tip is replaced by a special nano-TA thermal probe that has an embedded miniature heater and is controlled by the specially designed nano-TA thermal probe hardware and software. The AFM enables a surface to be visualised at nanoscale resolution with its routine imaging modes, which allows the user to select the spatial locations at which to investigate the thermal properties of the surface. The user then obtains this information by applying heat locally via the probe tip and measuring the thermomechanical response.
The results were obtained using an Explorer AFM equipped with a TA Instruments heat/cool stage and TP93 Linkham Temperature Controller, and the nano-TA thermal probe module (comprising controller, software and high resolution thermal probes). The nano-TA thermal probe data presented are of the probe cantilever deflection (whilst in contact with the sample surface) plotted against probe tip temperature. This measurement is analogous to the well established technique of thermo-mechanical analysis (TMA). Events such as melting or glass transitions that result in the softening of the material beneath the tip, produce a downward deflection of the cantilever. TMA experiments were performed in two different environments. In the case of the PVME/PS blends, the low glass transition temperature of the PVME forced us to work at reduced temperature (-30°C) under N2 atmosphere. In the case of the PP blends we worked under air at ambient temperature (30°C). Further information on the technique can be obtained at Anasys Instruments.
Results and Discussion
Semi Crystalline Blends
In the case of isotactic PP/syndiotactic Polypropylene (iPP/sPP) blends an Upper Critical Solution Temperature (UCST) type phase behaviour is predicted with liquid-liquid phase separation occurring prior to crystallisation in high molar mass blends. Figure 1a shows a 25/75 iPP/sPP blend with a fibrillar semi-crystalline morphology. The identification of the phase-separated morphology by traditional methods such as Optical Microscopy or TEM is very difficult due to a lack of contrast. Using nano-TA thermal probe, the phases can be identified and characterized by exploiting the difference in melting temperature (135°C for bulk sPP and 165°C for bulk iPP).
Figure 1. iPP/sPP 25/75 blend contact mode topographic image before (a) and after (b) nano-TA thermal probe (100 x 100 µm2). Image c is a 20 x 20 µm2 zoom on the analysed region showing seven local thermal analysis indents.
The transition temperatures were measured inside and in the vicinity of the fibrils. The indents resulting from the thermal analysis are seen in Figure 1b and 1c.
The data in Figure 2 show the deflection of the probe and are the average of multiple (5-10) measurements. First the thermal expansion of the sample deflects the probe upward, followed by the penetration of the probe tip into the polymer. The transition temperature is taken as the temperature at which the probe starts sinking into the material.
Figure 2. Nano-TA thermal probe measurement of an iPP/sPP 25/75 blend, showing the different melting temperature for the iPP and sPP domains.
The nano-TA thermal probe data on the fibrils (red curve) have a transition very close to bulk iPP while the surrounding material (green curve) shows a transition close to bulk sPP, so the material of each phase is clearly identified.
Poly(vinylmethylether)/polystyrene (PVME/PS) blends show a Lower Critical Solution Temperature behaviour (LCST). This means that starting from a homogeneous system, they phase-separate when heated above a certain temperature. In thin coatings, the influence of the air-polymer and substrate-polymer interfaces increases with decreasing film thickness and affects the phase-separation behaviour and the final morphology. This film thickness effect on the phase separation temperature is observed for films with a thickness below 1 µm and is substrate dependent. Thin films were studied on two different substrate chemistries, the natural oxide layer and a HF etched silicon surface.
Figure 3. Influence of the surface chemistry: Topographic (a, c) and friction (b, d) images of a phase separated PVME/PS 80/20 blend film on a Si (a, b) and SiOx (c, d) surface (10 x 10 µm2 scans).
AFM experiments were carried out at 30°C in contact mode, acquiring both topographic and lateral force images. At this temperature the PS rich phase is in the glassy state, which preserves the morphology formed at more elevated temperature.
After annealing above the LCST, a PVME rich phase and a PS rich phase are formed. Figure 3 illustrates the importance of the surface chemistry on the morphology.
Due to its higher surface tension it is assumed that the PS rich phase forms bumps and the PVME rich phase forms holes. Also, the holes have higher friction values, thus they probably correspond with the more viscous PVME rich phase. These indirect findings were confirmed by direct measurements of the glass transition temperature of the different domains with nano-TA thermal probe (Figure 4). Not only did the nano-TA thermal probe measurements give us direct lateral and in this case they also provided depth information, since a double penetration is observed in the case of the PS rich phase. This demonstrates that even the PS rich phase is covered by a thin layer of PVME, which is in accordance with XPS measurements in literature. It is also worth noting that PVME is the dispersed phase, although it is the majority component, while in contrast, the PS rich minority phase is acting as a framework.
Figure 4. Nano-TA thermal probe measurement of a PVME/PS 80/20 blend film on a SiOx surface, showing results for the PS Rich and the PVME rich phase.
Figure 5. Nano-TA thermal probe measurement indents in the different phases of a PVME/PS 80/20 blend film on a HF treated Si surface. Performed at -30°C under N2 atmosphere (10 x 10 µm2 scans).
Figure 5 illustrates the lateral resolution of the nano-TA thermal probe system. The mean radius of the seven indents equals 100nm. The size of the indents depends on the penetration depth and on the shape of the AFM tip.
Nano-thermal analysis in combination with AFM proves to be a very valuable tool for the study of polymeric coatings and surfaces in general, since it allows not only imaging but also direct identification and characterization of the different domains at the sample surface on a 100nm scale.
Source: Characterization of Blend Morphology of nanoscale Thin Films
Author: Nicolaas-Alexander Gotzen Ph.D. and Professor Guy Van Assche Ph.D
For more information on this source please visit Anasys Instruments