Phase separation and self-assembly in vitrimers: hierarchical morphology of molten and semi-crystalline polyethylene/dioxaborolane maleimide systems

Vitrimers - a class of polymer networks which are covalently crosslinked and insoluble like thermosets, but flow when heated like thermoplastics - contain dynamic links and/or crosslinks that undergo an associative exchange reaction. These dynamic crosslinks enable vitrimers to have interesting mechanical/rheological behavior, self-healing, adhesive, and shape memory properties. We demonstrate that vitrimers can self-assemble into complex meso- and nanostructures when crosslinks and backbone monomers strongly interact. Vitrimers featuring polyethylene (PE) as the backbone and dioxaborolane maleimide as the crosslinkable moiety were studied in both the molten and semi-crystalline states. We observed that PE vitrimers macroscopically phase separated into dioxaborolane maleimide rich and poor regions, and characterized the extent of phase separation by optical transmission measurements. This phase separation can explain the relatively low insoluble fractions and overall crystallinities of PE vitrimers. Using synchrotron-sourced small-angle X-ray scattering (SAXS), we discovered that PE vitrimers and their linear precursors micro-phase separated into hierarchical nanostructures. Fitting of the SAXS patterns to a scattering model strongly suggests that the nanostructures - which persist in both the melt and amorphous fraction of the semi-crystalline state - may be described as dioxaborolane maleimide rich aggregates packed in a mass fractal arrangement. These findings of hierarchical meso- and nanostructures point out that incompatibility effects between network components and resulting self-assembly must be considered for understanding behavior and the rational design of vitrimer materials

Fabrication, Characterization and Permeation Studies of Ionically Cross-linked Chitosan/Kaolin Composite Membranes

This paper presents the successful preparation of porous membranes based on chitosan with enhanced mechanical, thermal and chemical properties applicable in water treatment field. Herein, chitosan/kaolin composite membranes with a cross-linking agent and a porogen were prepared using the solvent casting method. The characterization of the as-fabricated membranes indicated that the combined effect of kaolin as reinforcing agent, polyethylene glycol as pore former and citric acid as cross-linker in a chitosan matrix showed a significant influence on the membrane properties. The results indicated that the incorporation of a hydrophilic porogenic reagent into the collodion in addition to providing a porous morphology makes it possible to obtain a more hydrophilic membrane, and thus induces an increase in the pure water permeability. The cross-linked membranes exhibited an improved water resistance, better thermal and mechanical properties as compared to neat chitosan films. The cross-linked membranes had a mean pore size of 50 nm falling in the range of ultrafiltration. Their functional properties were determined in terms of pure water filtration and molecular weight cut-off tests

The Multi-Stress Aging of 15 kV EPR Power Cables

This research is focused on the multi-stress aging phenomena and lifetime estimation of 15 kV EPR cable. In order to gain the suitable parameters for the lifetime estimation, the aging study on the EPR cable samples as well as on the cable layers’ dielectrics samples was carried out at the High Voltage Laboratory of Mississippi State University. During the multi-stress aging study of 15 kV EPR cable samples, the EPR cable samples underwent electrical stress, thermal stress, and environmental effects. The aging time for the EPR cables varied from 650 hrs to 1300 hrs. An empirical aging model describing the cables’ lifetime was derived from the partial discharge measurements results. The aging study on the EPR cable layers’ dielectrics was achieved as well. The EPR insulation material samples were aged by combined electrical and thermal stress, while the material samples of inner semi-conducting layer, outer semi-conducting layer, and outer low-density polyethylene (LDPE) jacket were aged by thermal stress. The measurement data was used for the newly proposed lifetime estimation method. A new lifetime estimation method was introduced for the EPR cables. The method assumed that the failures of cables results from the expansion of voids/cavities initiated from the defects in the EPR insulation layer. The proposed lifetime estimation method applied the finite element method (FEM) to solve the electric field distribution inside the EPR cable with the existence of voids/cavities. The parameters were derived from the aging study on the EPR insulation material samples. Assuming the voids/cavities would expand in the direction of the maximum electric field stress, the lifetime of the EPR cables was then estimated through the iteration. The introduced method helped to establish a relationship between the aging study of insulation material samples and the aging of EPR cable samples, which was long missing in the past studies. It also provided a new way to assess the reliability of the EPR cable

The Development of Polyurethane-based Solid-to-Solid Phase Change Materials for Thermal Energy Storage Applications

This work investigated and characterized the structure-property relationship of a polyurethane-based block copolymer and the thermal energy storage properties obtained through the solid-to-solid phase transition of a PCM polyol polymer that undergoes a thermal transition at low temperatures. The chemical and physical factors that influence or dictated the microphase separation between the urethane “hard” segment block and the polyol “soft” segment block of conventional polyurethanes how the resulting changes in phase morphology effects the crystallization behavior of the “soft” component, which in this dissertation is analogous to the PCM polymer component was analyzed. Theurethane HS group behaves as a cross-link and restricts PCM polymer chain mobility, thereby the PCM can no longer translate freely and instead exhibits a solid-to-solid phase transition. The introduction of HS cross-link exhibits a behavior known as the HS chain-end effect, in which HS constrictions cause the PCM polymer to become partially crystalline. The extent to which PCM crystallization is limited by the HS chain end effect can vary depending on HS structural factors, such molecular architecture and HS composition. The HS chain end effect was quantified for the following HS structural variables; HS cross-link nature, diisocyanate molecular geometry, HS chain length to characterize and compare how each factor limits PEG PCM polymer crystallization. By doing so in a systematic manner, the optimal configuration for an effective PU-SSPCM can be determined. To examine HS cross-link nature, an analogous linear and a non-linear PU-SSPCM polymer, were compared to determine differences in final thermal energy storage properties. The effects of diisocyanate molecular geometry was investigated by comparing the thermal energy storage properties of a series of PU-SSPCMs varied only by its diisocyanate component. The considered diisocyanates were selected based on specific structural moieties that affect the structural regularity, rigidity, and symmetry of the HS. The chain length of the hard segment component influence on thermal energy storage properties was also investigated by varying the proportions of urethane HS concentration and PCM polymer concentrations. HS cross-link nature, the HS diisocyanate component, and HS chain length are considered chemical level factors since they can be controlled during synthesis. On the physical level, the possibility of a connection between the degree of phase separation and thermal energy storage properties were explored. This relationship was investigated by measuring thermal energy storage properties of PU-SSPCMs with a high, medium, and low degree of phase separation. Varying HS crystallization by cooling rates from a homogeneous melt state was used to obtain different levels of phase separation. Thermal energy storage properties were measured using Differential Scanning Calorimetry experiments. Supplemental information about the chemical structure of the synthesized polyurethane-based solid-to-solid phase change materials (PU-SSPCM) was analyzed by FTIR analysis techniques. The phase morphology and the degree of phase separation prevalent in the prepared samples was characterized by FTIR, TGA, and DSC techniques

Molecularly controlled epoxy network nanostructures

AbstractEpoxy thermosets continue to be used in a variety of coatings, adhesives, and structural composites. Nanostructural heterogeneities have been proposed to determine the physical properties of these materials, but the presence and origin of these features is disputed. Here, we combine nano-chemical imaging and nano-thermal analysis to establish a connection between internal crosslinking and the appearance of nanoscale chemical heterogeneities in epoxy resins. Deflection of an AFM probe is used as a local sensor to detect photothermal expansion in response to infrared excitation, and nanoscale lateral variations are detected in response to illumination at wavenumbers associated with crosslinking. Furthermore, these heterogeneous chemical features correspond to an increased range of local thermal transitions, and only arise within highly cross-linked resins; lightly cross-linked specimens are found to be homogeneous

Silsesquioxane polymer as a potential scaffold for laryngeal reconstruction

Cancer, disease and trauma to the larynx and their treatment can lead to permanent loss of structures critical to voice, breathing and swallowing. Engineered partial or total laryngeal replacements would need to match the ambitious specifications of replicating functionality, outer biocompatibility, and permissiveness for an inner mucosal lining. Here we present porous polyhedral oligomeric silsesquioxane-poly(carbonate urea) urethane (POSS-PCUU) as a potential scaffold for engineering laryngeal tissue. Specifically, we employ a precipitation and porogen leaching technique for manufacturing the polymer. The polymer is chemically consistent across all sample types and produces a foam-like scaffold with two distinct topographies and an internal structure composed of nano- and micro-pores. Whilst the highly porous internal structure of the scaffold contributes to the complex tensile behaviour of the polymer, the surface of the scaffold remains largely non-porous. The low number of pores minimise access for cells, although primary fibroblasts and epithelial cells do attach and proliferate on the polymer surface. Our data show that with a change in manufacturing protocol to produce porous polymer surfaces, POSS-PCUU may be a potential candidate for overcoming some of the limitations associated with laryngeal reconstruction and regeneration