*2.4. PLA*/*Sorbitol Blends*

1,3:2,4-Bibenzylidene-d-sorbitol (DBS, Scheme 6), derived from the natural sugar alcohol d-glucitol, is considered an environmentally friendly material of acceptably low toxicity. It is a well-known low-molecular-weight supramolecular gelator of "butterfly-like" structure that has considerable potential in applications such as personal care products, adhesives, dental composites, gel electrolytes, or liquid crystalline materials and as a polymer nucleator [58].

**Scheme 6.** Chemical structure of d-sorbitol, 1,3:2,4-dibenzylidene-d-sorbitol (DBS), and 1,3,2,4-bis(3,4-dimethylobenzylideno)sorbitol (DMBS).

The benzylidene groups and hydroxyl moieties on the sorbitol backbone are two molecular recognition motifs underpinning the role of DBS in the formation of a nanoscale network and the gelation mechanism. It involves interactions between 5-OH/6-OH hydrogen-bond-donating groups of one molecule of DBS and the hydrogen-bond-accepting 5-OH/6-OH or the cyclic acetals of another molecule. The π-π stacking/solvophobic interactions between the aromatic substituents are other important factor in the molecular recognition pathway. In non-polar media, the intermolecular hydrogen bonding plays a key role in the self-assembly, whereas in polar, protic solvents, the π-π stacking and/or solvophobic interactions between the aromatic groups become more important. The primary one-dimensional nanostructures (nanofibrils) formed by DBS molecules bundle together into 3D nanofibers (10 nm to 0.8 μm in diameter) of helical or non-helical structure (depending on the environment's polarity), and a three-dimensional network is subsequently formed.

DBS can direct the crystallization of polylactide. It influenced the crystal structure of PLLA and a ffected its crystallization rate and melting behaviour. FTIR spectroscopy showed that the C=O stretching band in the PLLA sample at 1755 cm<sup>−</sup><sup>1</sup> became much broader as DBS was added, presumably due to hydrogen bonding between DBS and PLLA [52]. In the presence of DBS, the formation of the perfect α-form was favoured over the less ordered α'-structures [59]. The α-crystals were obtained more easily, and the disorder-to-order ( α'-toα) phase-transition was shifted to lower temperatures with an increasing concentration of DBS. This was explained in terms of supramolecular templating exerted by DBS nanofibrils. DBS molecules stacked together through π-π interactions to form a strand that was linked to PLLA macromolecules by hydrogen bonding (Scheme 7). The equilibrium melting point and glass-transition temperature of PLLA admixed with DBS were not significantly changed.

**Scheme 7.** Postulated interactions between the polylactide backbone and nanofibrils of self-assembled DBS [59]. Reprinted with permission from The Journal of Physical Chemistry B. Copyright (2011), American Chemical Society.

The X-ray di ffraction peaks at 2θ = 12.5◦, 14.7◦, 16.7◦, 19.1◦ and 22.4◦ correspond to the (103), (010), (200)/(110), (203) and (015) reflections characteristic of the α-crystals of PLLA, whereas the one at 2θ = 24.5◦ is representative of the less perfect α'-crystals. The latter are typically formed as a single form or coexist with α-crystals at crystallization temperatures <120 ◦C. It was found that with an increase in DBS content, the reflections (200)/(110) moved forward to the higher 2θ range and, contrary to the behaviour of neat PLLA, they were not significantly changed during isothermal crystallization at 120, 130 and 140 ◦C (Figure 8) [59]. The α-crystals predominate even at T < 120 ◦C. The temperature of the α'-toα phase-transition of PLLA was lowered with an increase in the DBS concentration, owing to the interactions between the polyester chains and the self-organized nanofibrils.

The DBS architectures could be tuned by varying the DBS contents and PLLA crystallization temperatures [52]. Micron-sized fibrillar rings or disks and "concentric-circled" PLLA spherulites were formed due to the aggregation of nanofibrils in samples with DBS contents >3 wt% crystallized above 120 ◦C (Figure 9). The dispersed nanofibrils a ffected the orientation of PLLA lamellae and caused a change in birefringence, not significantly a ffecting the spherulitic growth rate. If the local concentration of DBS was low then PLLA crystallized in a typical manner and DBS molecules were excluded outside the crystals. Their concentration in the amorphous region increased. When the PLLA crystallization was complete, the concentration of the DBS molecules ejected outside the PLLA crystals became high enough to let them self-assemble at the PLLA growth front and form nanofibrils between the spherulites. For higher DBS contents (3–4 wt%) the formation of nanofibrils was much easier. Therefore, they were formed during the process of PLLA crystallization and aggregated inside the spherulites. More regular PLLA crystals were formed at lower temperatures when larger amounts of DBS were added [60]. Moreover, the spherulitic growth rate of PLLA depended inversely on the fold surface free energy, which increased with the amount of the additive. DBS also enhanced the hardness and stiffness of the PLLA on cooling [61]. Furthermore, the hydrophilicity of the PLLA was significantly improved by an increase in the DBS concentration, which is important for biomedical applications of PLA. The crystallization of the PLA melt was also enhanced in a more complex blend, via the synergistic effect of DBS nanofibrils as the nucleating agent, combined with a plasticizer (poly(ethylene glycol), PEG) and a multifunctional monomer (pentaerythritol triacrylate, PETA) [62]. It was crucial to prepare the DBS/PEG gel before mixing those components with PLA. The mixing temperature was also critical. The acceleration of crystallization was ascribed to the increase in the nucleation density as well as the faster growth rate of spherulites in the presence of the plasticizer.

**Figure 8.** Wide-angle X-ray Scattering (WAXS) patterns of PLLA containing 0–4 wt% of DBS isothermally crystallized at (**a**) 90, (**b**) 100, (**c**) 110, (**d**) 120, (**e**) 130, and (**f**) 140 ◦C [59]. Reprinted with permission from The Journal of Physical Chemistry B. Copyright (2011), American Chemical Society.

**Figure 9.** Concentration-dependent formation of nanofibrils of DBS during crystallization of PLLA [52]. Republished with permission of Royal Society of Chemistry. Permission conveyed through Copyright Clearance Center, Inc.

An analogue of DBS—1,3,2,4-bis(3,4-dimethylobenzylideno)sorbitol (DMBS) (Scheme 6)—was applied to poly(lactic acid) as a nucleating and clarifying agen<sup>t</sup> [63]. The nucleated PLA crystallized earlier, and a reduction in the crystallization temperature was observed at DMBS concentrations of 0.25–10 wt%, while the glass transition temperature was decreased by ~10 ◦C. The composite maintained similar clarity at all DMBS concentrations. The tensile modulus and tensile strength increased slightly with the content of DMBS up to 1.5 wt% but dropped at higher concentrations of the additive. Biodegradable PLA composites were prepared using a combination of wood fiber (WF) and DMBS [64]. 1,3,2,4-bis(3,4-dimethylobenzylideno)sorbitol acted as an e ffective nucleating agen<sup>t</sup> for those composites, improved their thermal stability and mechanical properties, and slowed down their enzymatic degradation.
