**3.7 Physical modifications**

*Fillers - Synthesis, Characterization and Industrial Application*

transition temperature [24].

properties [50].

**3.6 Chemical modifications**

modified methylmethacrylate with acrylamide [31].

heat resistance, enhanced the barrier properties to water vapor, and lowered glass

In PVA/starch biohybrid film, natural raw materials were also added as fillers such as cellulose nanofibers, chitosan, and feather keratin. In investigations, cellulose nanofibers blocked the recrystallization of starch by decreasing the mobility of polymer chains. Hence, the physicomechanical properties and crystal structure of blend film were significantly enhanced. In relation to this, storage conditions of biobased films improved. Cellulose nanofibers significantly enriched also the stiffness and strength of blend films by the storage conditions [34]. Similarly, the storage conditions of biobased films in natural weathering could be also enhanced by added graphene into PVA and starch [45]. Moreover, when the chitosan known as a natural filler was added to biofilms, their physicochemical properties such as TS, E%, water vapor permeability, and oxygen transmission rate improved. Also, water vapor and oxygen permeability, water uptake, and hydrophobic character of the chitosan-added bioblend film were better than biofilm without chitosan due to its incorporation [46]. As different inorganic salts are used in the biobased films, their crystalline [47], thermal, water vapor barrier, and mechanical properties can be significantly affected via strong hydrogen bonds. For this reason, Jiang et al. have reported that LiCl, MgCl2,6H2O, CaCl2, and AlCl3,6H2O salts have provided a good compatibility with PVA and starch [48, 49]. Moreover, the ZnO added biobased films have shown good dispersion, homogeneity, mechanical properties, and water resistance [49]. Another filler salt, AlCl3.6H2O, can show compatibility with PVA and starch. Hence, these salts have presented great destroying effect on the crystalline and good mechanical

The chemical modifications applied to the biohybrid films produced from PVA and starch have improved their physical-biological-chemical properties because of the changing molecular structure of blend. For instance, the carboxyl group of PVA and starch has occurred in bioblend films after oxidation of starch with H2SO4 and KMnO4. After the increase of polar carboxyl groups by oxidation, the hydrogen bonds in blend molecules were stronger than those of nonmodified ones. At the same time, their TS and E% have improved [44]. After modification, hydroxypropyl distarch phosphate converted from starch has shown highest TS and capability of retarding evaporation of water due to being compatible with fillers [4]. In another modification, PVA and starch blend grafted with methylmethacrylate had a higher E% and water desorption. For this reason, polysaccharide chain of starch and OH- groups of PVA are mostly occupied with monomers [51]. However, Yoon et al. depicted that TS increased on the contrary E% after using to blend film

The plasma and irradiation treatment known as novel modification were also applied to PVA and starch blend films. These treatments can influence physicochemical properties of biohybrid films. Therefore, the treatment can cause a chemical bonding or graft functional groups on the PVA and starch backbone without any additives [52, 53]. Hence, the carbonyl groups of biofilms are improved with plasma treatment by using rotary argon plasma equipment. In addition, while E% of blend films can tolerate, its TS could also be lower. The plasma or irradiating pretreated with PVA and starch exhibited better thermal, processing, and mechanical (tensile) properties and toughness due to the induction of the cross-linking reaction [53]. With irradiated or plasma modifications, biofilms could prolong the storage condi-

**10**

tions up to 15 days [54].

Physical modification of PVA and starch could be safely used in biohybrid films. In generally, gelatinized, ungelatinized, fast and slow drying, varying amylose contents of raw material starch, changing of PVA and starch ratio, and impregnation of antioxidants were preferred in blend films for this modification. Applying physical modifications to biobased films affects significantly their physicomechanical properties. For example, gelatinized starch-polyvinyl alcohol blend films illustrate their uniformity of morphologies than the ungelatinized films, which corresponds well with the intensity of newly formed hydrogen bonds between starch, polyvinyl alcohol, and plasticizer. After the starch was gelatinized, the melting point of blend film decreased because of forming stronger hydrogen bonding interactions at an elevated preparation temperature [11]. At the same time, the gelatinized procedure is believed as a useful way to eliminate the crystalline structure [55]. In the fast (at 50°C) and slow (at 5°C) drying modification of the blend films, solubility, TS, E%, and degree of swelling values of the biobased films are preferable at slow drying than those of fast drying owing to the hydrogen bonding interaction forming at low temperature [56].

With increasing amylopectin contents of starch in blend film, the %E and Tg increased while TS decreased. The linear structure of amylose improved the tensile property of films (especially, the amount of elongation) and the degree of crystallinity. However, because of the amorphous structure of amylopectin, %E of blend film was lower. Increasing amylose ratio in blend film significantly increased %E values as it plays an important role in cross-linking [7]. Moreover, changing rates of PVA and starch illustrated important role in blend films. The TS of the film decreased with increasing starch content in PVA compared to pure PVA film [5]. At the same time, the crystallinity of PVA in blend film decreased importantly compared to pure PVA film. On the other hand, the water absorption of blend film increased with the increasing starch ratio, because the water absorption of PVA is weaker than that of starch [29].

The impregnation of antioxidants into the biofilm is another physical modification technique bringing antioxidative effect in biofilms. For instance, PVA/starch biofilms impregnated with catechin showed antioxidant and antimicrobial properties, while TS and %E of films decreased. Moreover, the biofilm containing catechin hinders lipid oxidation and microbial growth on raw meat during storage condition without substantial change in redness compared with commercial polyethylene pack [57].
