Model-Independent X-ray Reflectivity Fitting for Structure Analysis of Poly(3-hexylthiophene) Films

Interfacial Engineering of Soft Matter Substrates by Solid-State Polymer Adsorption

Polymer coating to substrates alters surface chemistry and imparts bulk material functionalities with a minute thickness, even in nanoscale. Specific surface modification of a substate usually requires an active substrate that, e.g., undergoes a chemical reaction with the modifying species. Here, we present a generic method for surface modification, namely, solid-state adsorption, occurring purely by entropic strive. Formed by heating above the melting point or glass transition and subsequent rinsing of the excess polymer, the emerging ultrathin (<10 nm) layers are known in fundamental polymer physics but have never been utilized as building blocks for materials and they have never been explored on soft matter substrates. We show with model surfaces as well as bulk substrates, how solid-state adsorption of common polymers, such as polystyrene and poly(lactic acid), can be applied on soft, cellulose-based substrates. Our study showcases the versatility of solid-state adsorption across various polymer/substrate systems. Specifically, we achieve proof-of-concept hydrophobization on flexible cellulosic substrates, maintaining irreversible and miniscule adsorption yet with nearly 100% coverage without compromising the bulk material properties. The method can be considered generic for all polymers whose Tg and Tm are below those of the to-be-coated adsorbed layer, and whose integrity can withstand the solvent leaching conditions. Its full potential has broad implications for diverse materials systems where surface coatings play an important role, such as packaging, foldable electronics, or membrane technology.

Structural Order in Cellulose Thin Films Prepared from a Trimethylsilyl Precursor

Biopolymer cellulose is investigated in terms of the crystallographic order within thin films. The films were prepared by spin-coating of a trimethylsilyl cellulose precursor followed by an exposure to HCl vapors; two different source materials were used. Careful precharacterization of the films was performed by infrared spectroscopy and atomic force microscopy. Subsequently, the films were investigated by grazing incidence X-ray diffraction using synchrotron radiation. The results showed broad diffraction peaks, indicating a rather short correlation length of the molecular packing in the range of a few nanometers. The analysis of the diffraction patterns was based on the known structures of crystalline cellulose, as the observed peak pattern was comparable to cellulose phase II and phase III. The dominant fraction of the film is formed by two different types of layers, which are oriented parallel to the substrate surface. The stacking of the layers results in a one-dimensional crystallographic order with a defined interlayer distance of either 7.3 or 4.2 Å. As a consequence, two different preferred orientations of the polymer chains are observed. In both cases, polymer chain axes are aligned parallel to the substrate surface, and the orientation of the cellulose molecules are concluded to be either edge-on or flat-on. A minor fraction of the cellulose molecules form nanocrystals that are randomly distributed within the films. In this case, the molecular packing density was found to be smaller in comparison to the known crystalline phases of cellulose.

Solvent dependent ordering of poly(3-dodecylthiophene) in thin films

The strong influence of solvents on the ordering of poly(3-dodecylthiophene) (P3DDT) due to edge-on oriented stacking, in the spin-coated thin film on the Si substrate, both near the substrate and away from it, depending upon the substrate surface nature, is observed from the X-ray reflectivity study. The absence of any appreciable amount of coil-like P3DDT chains (i.e. charge localized states) and formation of π-stacked aggregates (i.e. charge delocalized states) in the spin-coated thin films, with slightly better uniformity for the film prepared from toluene (TL) compared to that prepared from chloroform (CF) and chlorobenzene (CB), are well evident from the optical absorption study. No ordering near the weakly hydrophobic H-Si substrate is found in the films prepared from TL, probably due to less diffusion of P3DDT in TL and the appreciable pinning (film-substrate interaction) effect, while appreciable ordering near the film-air interface, overcoming the pinning effect, is likely to be related to the moderate values of the viscosity and the evaporation rate of the solvent. A better ordered Form-I-like relaxed structure near the film-substrate interface and a less ordered interpenetrating Form-II-like structure toward the film-air interface are found in the films prepared from CF, probably related to the low viscosity and high evaporation rate, respectively, of the solvent. Less ordered and mixed but more toward Form-II-like structures are formed throughout the film prepared from CB, probably due to the high viscosity of the solvent, even though its evaporation rate is low. The high evaporation rate of CF and high viscosity of CB probably create hindrance in the formation of continuous films on the weakly hydrophilic O-Si substrate at low speed, while the moderate values of both the parameters for TL, help to form continuous films on the O-Si substrate even at low speed. Such moderate values also help to form less variable (and more toward Form-I-like) structures and better ordering in the latter film. The relative fluctuation between aggregates along the film-thickness is, however, found slightly more in the film prepared from TL compared to that prepared from CF.

Dissolution testing of hardly soluble materials by surface sensitive techniques: clotrimazole from an insoluble matrix

PURPOSE

The low aqueous solubility of many drugs impedes detailed investigation as the detection limit of standard testing routines is limited. This is further complicated within application relevant thin films typical used in patches or stripes for buccal or topical routes.

METHODS

In this work a model system is developed based on spin - casting technique allowing defined clotrimazole and clotrimazole - polystyrene composite films preparation at a solid surface. Various highly sensitive techniques including quarz crystal microbalance (QCM), X-ray reflevtivity (XRR) and X-ray photon spectroscopy (XPS) are used to investigate the drug release over time into an aqueous media.

RESULTS

The results reveal a steady drug release for both samples over the course of the experiments but with the release from the composite being significantly slower. In addition the dissolution rate of the clotrimazole sample initially increases up to 30 min after which a decrease is noted. XRR shows that this is a result of surface roughening together with film thickness reduction. The results for the composite show that the release in the composite film is a result of drug diffusion within the matrix and collapsing PS film thickness whereby XPS shows that the amount of clotrimazole at the surface after 800 min immersion is still high.

CONCLUSION

It can be stated that the applied techniques allow following low mass drug release in detail which may also be applied to other systems like pellets or surface loaded nano-carriers providing information for processing and application relevant parameters.

Non-contact-mode AFM induced versus spontaneous formed phenytoin crystals: the effect of layer thickness

In this work the model substance phenytoin is vacuum deposited onto silica substrates resulting in amorphous films which are transferred via a non-contact AFM method into crystalline phenytoin.