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Herein ultralow density aerogels are developed through a feasible freeze casting technique. Briefly, the syndiotactic polystyrene (sPS) gel was prepared using xylene and subjected to successive steps of solvent exchange using non-solvents like ethanol and water. These samples were freeze-dried to obtain aerogels of the same shape and size as gels. The aerogels obtained exhibited the lowest ever dielectric constant of any polymer aerogel (κ = 1.03 ± 0.02), which is very close to that of air. Moreover, while analyzing the crystal structure, it was observed that the water derived sPS aerogel existed in the empty δ (δe) form, with microcavities in the crystal lattice in addition to meso and macroporosity. However, annealing at different temperatures yielded aerogels with different polymorphic forms without microporosity leading to slight variations in the dielectric constant. The microporosity in the crystalline state along with the lower density of the material were proven to be the major contributing factors to the lowest ever reported dielectric constant of the sPS aerogel. The sPS aerogels with an ultra low dielectric constant, very high hydrophobicity and adequate mechanical strength can stand as an alternative to conventional brittle inorganic aerogels.

The role of a nonsolvent in controlling the crystallization and morphology of solvent-crystallized poly(l-lactide) (PLLA) films was investigated using various microscopy techniques and small- and wide-angle X-ray scattering (SAXS/WAXS). PLLA films crystallized in THF and acetone had 40-80 μm spherulites. When water was present in the solvent, a completely different morphology was observed with nanosized voids and the surfaces of the films were smooth. In contrast, SEM studies revealed that the films crystallized in acetone and THF which had macroporous structures, had larger voids and film surfaces were rough because of the presence of globular structures. Voids appeared within the spherulites in the THF/water treated film, whereas crystals nucleated at the surface of the nanosized voids in acetone/water treated PLLA films. The formation of such voids is attributed to the interface-enhanced crystal nucleation in a solvent/nonsolvent system where the nonsolvent increases the polymer crystal nucleation and the subsequent evaporation of the nonsolvent. The method described in this work can be extended to other polymers to control the morphologies of polymer films during solvent-induced crystallization.

Frontiers of bio-based and biodegradable polymers are constantly expanding in a view to achieve sustainability. Hence, designing sustainable bioplastics made of either bio-based or biodegradable polymers opens up opportunities to overcome resource depletion and plastic pollution. This review presents a broad perspective on state-of-the-art technologies in bioplastics manufacturing along with the challenges underlying their production, application and post-consumer waste management. Recent scientific advances are catalysing the sustainable design of bioplastics to overcome the present challenges of plastic waste and emerging end-of-life options are contributing to circular economy. As research insights into developing sustainable bioplastics are rapidly evolving, their production and waste management approaches are not limited to those discussed in this review.

Differently functionalized boron nitride nanosheets (BNNSs) [hydroxyl (OH_BNNSs), amine (NH2_BNNSs), and poly(ethylene glycol) (PEG) (PEG_BNNSs)] were synthesized, and their effects on the structure and thermal properties of poly(l-lactic acid) (PLLA) along with those of the pristine BNNSs were studied. Highly dispersed nanocomposites were prepared using PLLA and 0.5 wt % of pristine/functionalized BNNSs via a solvent blending method. Homogeneous dispersion of BNNSs in the polymer matrix was confirmed using X-ray diffraction and scanning electron microscopy. Pristine BNNSs and OH_BNNSs accelerated the crystallization of PLLA as effective nucleating agents and favored the formation of the α form in melt-crystallized samples. On the other hand, NH2_BNNSs and PEG_BNNSs incorporated samples result in the moderate crystallization rate of PLLA and lead to the formation of a mixture of α and α' forms similar to the PLLA. It is also found that thermal stability and thermal conductivity of PLLA nanocomposites significantly depend on the type of functionalization of BNNSs. At 0.5 wt % loading, the thermal conductivity enhancement is maximum for PEG_BNNSs incorporated PLLA (∼62%), and that is only 9% for pristine BNNSs incorporated PLLA. The thermal stability of PLLA nanocomposites was significantly improved by 32-41 °C depending on the type of functionalized BNNSs compared to PLLA. It is proposed that the strong interaction between functionalized BNNSs and PLLA matrix is responsible for the improved thermal management properties.

Summary In this work, the effects of initial morphology/crystalline form and the guest molecules used for the preparation of syndiotactic polystyrene (sPS) co-crystals, on the structural phase transitions of sPS upon the guest extraction process were systematically investigated. For that purpose, samples having two different initial morphologies such as amorphous and γ form were used as the starting materials and guests such as chloroform and toluene were used for the preparation of sPS co-crystals. Selective extraction of guest molecules resulted in complex phase transitions in the host structure. In the case of toluene-treated amorphous samples, the structure transformed into the δ (δe) form. On the other hand, the chloroform-treated amorphous samples transformed into the mixture of nanoporous δ (δe) and ϵ (ϵe) forms. When chloroform-treated γ form samples were used for the guest extraction, the ϵ (ϵe) form with channel-shaped cavities was obtained. In contrast, when toluene-treated γ form samples were for used guest extraction, the co-crystal structure transformed to its initial crystalline form, i.e., γ form, instead of forming nanoporous structure.

Polymer–solvent complexes, poly(l-lactide) (PLLA) with cyclopentanone (CPO), were studied at multiple length scales using differential scanning calorimetry, small-angle neutron scattering, Fourier transform infrared spectroscopy, and temperature-dependent wide- and small-angle X-ray scattering. PLLA crystallizes in the e form when organic solvents such as CPO are incorporated into the crystal lattice at subambient temperatures. The transformation of this structure into the α form during solvent desorption and the accompanying changes in the lamellar structure were followed by various measurements on PLLA/CPO cocrystals. SANS data suggest that CPO is present stoichiometrically in the crystal lattice and as clusters in the interlamellar amorphous regions in the nominally dried samples. DSC thermogram showed a sharp endotherm during this e to α transition. X-ray fiber diagrams showed that the e form transforms to the α form over a temperature range (40–55 °C) as the solvent molecules are expelled from the cr...

The role of host preparation method on the structural phase transition of syndiotactic polystyrene (sPS) after the guest exchange process has been thoroughly investigated in our previous study using a series of n -alkanes [Polymer 2013; 54:6617-27]. In addition to the host preparation method, we have found that guest molecules used for the guest exchange process also have a significant role in facilitating the phase transitions of the host structure. Chloroform and toluene treated γ form samples were used as starting materials for the guest exchange process with different kind of solvents. Solvents that are capable of crystallizing sPS into the delta ( δ ) co-crystal are termed as clathrate guests and the solvents which are crystallizing sPS directly into the γ form are termed as γ form inducing solvents. Upon guest exchange process (performed by dipping the samples in clathrate guests and γ form inducing solvents), chloroform treated of γ form ( e clathrate ) sample transformed into the mixture of e clathrate and δ clathrate in the presence of clathrate guests, whereas the e co-crystal is retained its structure in the presence of γ form inducing solvents. However, the toluene treated γ form (monoclinic δ ) samples show different structures upon guest exchange process depending on the kind of guest molecules. In the case of clathrate guests, upon the guest exchange, the monoclinic δ retained its structure; however, the toluene treated γ form (monoclinic δ ) transformed to the γ form in the presence of γ form inducing solvents. Apart from the guest exchange process, structural changes upon the guest extraction process were also investigated.