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Materials Research in the Chen lab

Crystallization

Many molecules can form different crystal structures depending on the polymer substrate used to initiate crystallization. These polymorphs are useful in many applications, such as pharmaceuticals. We are working to explore the structure of drug molecules at buried interfaces by exploring the interactions between drug crystals and polymer substrates, for the rational control of drug polymorph growth.

Polymer Surfaces

1. Biomedical polymer surfaces

The performance of many polymers is dominated by surface/interfacial properties, such as wettability, friction, adhesion, biocompatibility, chemical reactivity, and permeability. These properties are widely used in many modern technological fields, and depend critically upon details of the molecular structures at the polymer surface or interface. We are studying the molecular surface structures of a variety of biomedical polymer materials (including polyurethanes, silicones, various polymer blends, and copolymers) using SFG, AFM, and other analytical techniques. The surface structures of chemical vapor deposited polymer coatings are also being studied for possible biomedical applications.

2. Marine anti-biofouling coatings

We are examining the molecular surface structures of new polymer coating materials developed for marine anti-fouling and fouling release. Such materials include variants of PDMS, including commercially available samples and polymers modified to incorporate various biocides (such as triclosan and ammonium salts). We also study the surface structures of zwitterionic polymers, polymers incorporated with fluorinated side chains, and polymer blends with micro- and nano-domain structures.

3. Restructuring behavior of polymer surfaces in water

Polymers are widely used in wet environments, such as biomedical polymers and marine anti-biofouling coatings- therefore, it is very important that we understand the responses of polymer surfaces to liquids. Most surface sensitive techniques require a high vacuum to operate, and thus cannot be applied to study surfaces in water. However, SFG is well suited to these environments. In addition to investigating surface restructuring behaviors of biomedical polymers and marine anti-fouling/fouling release coatings in water, we also have extensively studied the surface restructuring behaviors of model polymers (such as polymethacrylates) in water. A method has been developed to quantitatively deduce changes in the polymer surface structure in the presence of water.

Surface restructuring in water

 

Adhesion

1. Polymer-silane interactions

Silane adhesion promoters are well suited for metals and other inorganic substrates that have reactive hydroxyl functional groups at their surfaces. Research shows that alkoxysilane-based adhesion promoters can be quite effective in addition-curing silicones for imparting adhesion to some engineering thermoplastics, but the molecular mechanisms for such adhesion are not known. We study molecular interactions between various polymers and different silane molecules. Head groups, chain lengths, and end groups of silanes are systematically varied in the research. Particularly, a mixture of an epoxy silane and oligomeric siloxane, which is used as an effective adhesion promoting system is being investigated.

JPC B Cover story- Polymer silane interactions

2. Silicone Adhesives

Plastics are increasingly being used in many applications as mentioned above. For such applications, there is an increasing demand for silicone-based materials that can adhere to plastics, without surface priming or pre-treatment. Silicone elastomers are widely used because of their unique rheological properties, purity, and unsurpassed thermal stability and flexibility at a wide range of temperatures. In particular, hydrosilylation- or addition-cured silicones offer advantages due to their clean cure chemistry and controllable and rapid cure kinetics. However, unlike condensation-cure reactions, hydrosilylation-cured silicones are not naturally enriched with polar, moisture-reactive groups and generally require the addition of adhesion promoting additives. Hence, adhesion promoters are necessary to render these elastomers self-adherent. In addition to studying molecular interactions between various silane molecules and polymer surfaces, structural information of silane molecules or silane mixtures with oligmeric siloxane will be investigated at the buried polymer/polymer interfaces. Traditionally, it is necessary to break the interface and expose two resulting surfaces to air for investigation. The structures of the two resulting surfaces may not be related to that of the buried interface, especially for a well adhered interface. In this research, SFG is being used to probe structural information about an adhesion promoting system at the buried polymer/polymer interfaces in situ. We will also measure adhesion of the interfaces using a high-throughput adhesion testing instrument.

3. Polymer/metal interactions

It is necessary to develop robust adhesion between polymers and metal surfaces in many applications such as anti-corrosion coating for metal materials and polymeric adhesives (e.g., underfills) for microelectronics. In this research, we combine SFG and surface enhanced Raman spectroscopy (SERS) to study thin polymer films deposited on various metal surfaces and SAMs on metals surfaces. Metal surface roughness has been systematically varied.

4. Molecular dynamics simulations

We are carrying out molecular dynamics simulations to examine the molecular interactions between various polymer surfaces and silane molecules. Such results will be correlated to our experimental data, providing in-depth understanding of polymer-silane interactions.

5. Nano-diffusion

Our research indicates that some silane molecules can slowly diffuse through various polymer materials. We measure the kinetics of such slow diffusion using SFG.

SFG reveals changes in silane ordering

Conductive polymers

Conducting polymers are widely applied in many applications including various solar cells. Surface and interfacial structures play important roles in such applications, but have not been elucidated due to the lack of analytical tools. We are investigating molecular surface and interfacial structures of various conducting polymers, such as polythiophene derivatives and PEDOT.