How the Aggregates Determine Bound Rubber Models in Silicone Rubber? A Contrast Matching Neutron Scattering Study

The correlation between aggregates and bound rubber structures in silicone rubbers(S(phr)) with various silica fractions(ΦSi) has been investigated by contrast matching small-angle neutron scattering(SANS), swelling kinetics, and low-field nuclear magnetic resonance(NMR).Mixed solvents with deuterated cyclohexane fractions of 4.9% and 53.7% were chosen to match the scattering length densities of the matrix(SMP(phr)) and the filler(SMS(phr)), respectively. All the data consistently suggest that:(i) There is a critical threshold ΦSic between 10 and 30 phr;below ΦSic, the isolated aggregates are dominant, while beyond ΦSic, some rubber fraction is trapped among the agglomerate;(ii) ΦSiindependent thicknesses around 7.5 nm(NMR) and 8.6 nm(SANS) suggest that the bound rubber formation is determined by inherent properties of the components, and the power-law around 4.2 suggests an exponential changed gradient density of the bound rubber;(iii) SMS(80) presents a bicontinuous bound rubber with three characteristic lengths of 41, 100, and 234 nm. The expanded correlation length, a 20 nm smaller aggregate sizes suggest that such existent bicontinuous network in dry samples with less ΦSi is kind of impacted by swelling. With the obtained bound rubber models, the reinforcing mechanism of filled silicone rubber is elucidated.     

Quantitative Investigation on Structural Evolution of Co-continuous Phase under Shear Flow

Components of co-continuous phase can form an interpenetrating network structure, which has great potential to synergistically improve the mechanical properties of the blends, and to impart the functional blends superior electrical conductivity and permeability. In this work, the effects of shear rates (50–5000 s^?1) at different temperatures on the phase morphology, phase size and lamellar crystallites of biodegradable co-continuous polybutylene terephthalate (PBAT)/polybutylene succinate (PBS) blend are quantitatively investigated. The results show that the above features of the PBAT/PBS have a strong dependence on the shear flow and thermal field. The co-continuous phase of the blend is well maintained at 130 °C. Interestingly, this phase structure transforms into a “sea-island” structure at 160 °C, which gradually recovers to a co-continuous phase when the shear rate increases from 1000 s^?1 to 5000 s^?1. The phase size decreases with the increase of shear rate both at 130 °C and 160 °C due to the refinement and deformation of phase structures caused by strong shear stress. Unexpectedly, a unique phenomenon is observed that the shear-induced lamellar crystallites are oriented perpendicular to shear direction in the range of 500–5000 s^?1 at 130 °C, while the orientation of lamellar crystallites at 160 °C is along the shear direction within the whole range of shear rates. The degree of orientation for the PBAT/PBS blend crystals increases first and then decreases at both temperatures above. In addition, the range of shear rate has reached the level in the industrial processing. Therefore, this work has important guiding significance for the regulation of the co-continuous phase structure and the performance for the blend in the practical processing.

     

Phase assembly-induced transition of three dimensional nanofibril- to sheet-networks in porous cellulose with tunable properties

Ultralight and highly porous cellulose was fabricated via cellulose/sodium hydroxide/urea aqueous solution followed by gelation, coagulation and freeze-drying in the current work. The water content and freeze rate of cellulose coagulated sample are two crucial factors controlling the morphology, density and porosity of porous cellulose, which led to an interesting morphological transition from three dimensional nanofibrillar network to sheet network in porous cellulose. It was proposed that the aggregation and assembly of cellulose-rich phase and crystallization of water-rich phase were closely related to this transition. Based on this concept, a series of cellulose materials with densities varied from 0.129 to 0.330 g cm^?3 and corresponding porosities ranged from 91.4 to 78.0 %, were obtained. The porous celluloses showed a good ductility (strain to fracture is more than 30 %) and high modulus, which also could be tuned by porous morphology. The new understanding on the morphological transition in porous cellulose could be beneficial for the development of “green” porous materials.     

Interfacial Banded Transcrystallization of Polyoxymethylene/Poly(butylene succinate) Blends Induced by the Polyamide 6 Fiber

Interfacial crystallization of polyoxymethylene/poly(butylene succinate) blends induced by the polyamide 6 (PA6) fiber was investigated. Due to strong heterogeneous nucleating ability, dense nuclei were generated on the surface of the PA6 fiber, which compelled the growth of twisted lamellae perpendicular to the PA6 fiber. As a result, unique interfacial banded transcrystallization was formed, which is rarely found before. Crystallization temperature was dominant in determining the nucleation activity of the PA6 fiber, further affecting the architecture of banded transcrystallization. With the increase of crystallization temperature, the nucleation density decreased to give more growth space for the twisted lamellae around the fiber. The wave-like banded stripes were transformed into fan-like stripes. Accordingly, band spacing and eccentricity respectively showed positive and negative correlation with crystallization temperature. These meaningful results shed light on regulating the architecture of banded crystals in polymer composites.