Thermal Analysis, Structural Studies and Morphology of Spider Silk-like Block Copolymers.
Abstract: Spider silk is a remarkable natural block copolymer, which offers a
unique combination of low density, excellent mechanical properties, and thermal stability
over a wide range of temperature, along with biocompatibility and biodegrability. The
dragline silk of Nephila clavipes, is one of the most well understood and the best
characterized spider silk, in which alanine-rich hydrophobi... read morec blocks and glycine-rich
hydrophilic blocks are linked together generating a functional block copolymer with
potential uses in biomedical applications such as guided tissue repair and drug delivery.
To provide further insight into the relationships among peptide amino acid sequence, block
length, and physical properties, in this thesis, we studied synthetic proteins inspired by
the genetic sequences found in spider dragline silks, and used these bioengineered spider
silk block copolymers to study thermal, structural and morphological features. To obtain a
fuller understanding of the thermal dynamic properties of these novel materials, we use a
model to calculate the heat capacity of spider silk block copolymer in the solid or liquid
state, below or above the glass transition temperature, respectively. We characterize the
thermal phase transitions by temperature modulated differential scanning calorimetry
(TMDSC) and thermogravimetric analysis (TGA). We also determined the crystallinity by TMDSC
and compared the result with Fourier transform infrared spectroscopy (FTIR) and wide angle
X-ray diffraction (WAXD). To understand the protein−water interactions with
respect to the protein amino acid sequence, we also modeled the specific reversing heat
capacity of the protein-water system, Cp(T), based on the vibrational, rotational and
translational motions of protein amino acid residues and water molecules. Advanced thermal
analysis methods using TMDSC and TGA show two glass transitions were observed in all
samples during heating. The low temperature glass transition, Tg(1), is related to both the
bound water removal induced conformational change and the hydrophobicity of the protein
sequences, while the high temperature glass transition, Tg(2), above 130 °C is the now dry
protein glass transition. Real-time Fourier transform infrared spectroscopy (FTIR)
confirmed that conformational change occurred during the two glass transition, with a
random coils to beta turns transition during Tg(1) and alpha helices to beta turns
transition during Tg(2). Due to the hydrophobic and hydrophilic nature of the blocks, the
spider silk block copolymers tend to self-assemble into various microstructures. To study
the morphological features, the spider silk-like block copolymers were treated with
hexafluoroisopropanol or methanol, or subjected to thermal treatment. Using scanning
electron microscopies, micelles were observed in thermally treated films. Fibrillar
networks and hollow vesicles were observed in methanol-cast samples, while no
micro-structures were formed in HFIP-cast films, indicating that morphology and
crystallinity can be tuned by thermal treatments. Results indicate when we increase the
number of repeating unit of A-block in the protein, sample films crystallize more easily
and are more thermally stable. Moreover, when samples crystallize, the secondary structure
of A-block and B-block become different, thus it will be easier to form bilayer structures
which could fold into vesicles or tube structures during drying.
Thesis (Ph.D.)--Tufts University, 2013.
Submitted to the Dept. of Physics.
Advisor: Peggy Cebe.
Committee: Roger Tobin, Cristian Staii, Gary Goldstein, and Christoph Schick.
Keywords: Physics, and Biomedical engineering.read less