Designing, Preparation And Characterization Of Starch-based Self-reinforced Iflms

Starch is one of the most promising natural polymers because of its inherentbiodegradability, overwhelming abundance and its ongoing annual renewal. Improvements inmechanical properties and processability are two ongoing challenges in starch-based materialssince they have relatively weak mechanical properties and poor processability. In order toimprove the mechanical properties and processability of starch-based materials, a newtechnique of designing and preparation of self-reinforced starch flms is introduced anddiscussed in this thesis.The hydroxypropyl groups introduced into the starch chains are capable of disrupting inter-and intra-molecular hydrogen bonds, thereby weakening the granular structure of starchleading to an increase in motional freedom of starch chains in the amorphous regions, sohydroxypropylation was used to decrease gelatinization temperature and improveprocessability in this work. On the other hand, cross-linking reinforces the hydrogen bonds inthe granule with chemical bonds which act as bridges between molecules. When thecrossed-linked starch is heated in water, the granule will remain intact to various degrees bythe chemical bridges. The reinforcing component consisted of cross-linked starch granules,where the crosslinking increased granule thermal stability and moisture resistance. In thiswork, a high amylose cornstarch (amylose content80%) was used as the matrix that have beenchemically modifed by hydroxypropylation to improve its thermal behavior, leading to adecrease of gelatinization temperature and improving processability. Cross-linked starchgranules with improved thermal stability and water resistance was used to reinforce the matrixflm prepared by the hydroxypropylated starch. The key results are showed as following:1. The morphologies and thermal properties of a high amylose cornstarch modifed byhydroxypropylation were studied by polarization microscope and differential scanningcalorimetry (DSC). It was found that, gelatinization endotherm G became weaker andshifted to lower temperature range with increasing level of MS, endotherm M2attributedto the melting of amylose–lipid complexes was disappeared after hydroxypropylation. The endotherm Gr is bigger for native starch and decreased with decreasing MS. Some crackswere detected on the surface of high amylose hydroxypropylated starch granules, and thebirefringence of the granules decreases somewhat with increasing hydroxypropylation.2. The thermal properties, macromolecular weight and the morphologies of a high amylosecornstarch modifed by cross-linking were studied by DSC, gel permeationchromatography coupled with multi-angle laser light scattering (GPC-MALLS) and SEM.It was found that, the gelatinization temperature was increased with the increasingphosphorus content; the molecular weight of cross-linked starch was higher than nativestarch. As the phosphorus content increased to a certain degree the cross-linked starchcould not dissolve in DMSO solusion. There is no change in the appearance of the starchgranule with lower degree of cross-linking. The surface of the cross-linked starch granuleswere smooth and similar to that of the native starch with lower cross-linking, but as thephosphorus content was higher, the edge of starch granules become sharped.3. Self-reinforced starch films were firstly prepared in this work. Both hydroxypropylatedstarch and cross-linked starch were modified from high amylose corn starch. They wereused as the matrix and the reinforcement respectively. SEM, Confocal laser scanningmicroscopy (CLSM) and Instron tensile testing apparatus were used to study the theinterface between matrix and particle of the starch films; the distribution of thecross-linked starch and mechanical properties of the starch flm. It was found that, theparticles of cross-linked starch can be clearly identifed on the flm surface after drying andshrinking of the flmmatrix. The interface is smooth and no gap between the matrix andparticles were observed. The granules of cross-linked starch distribute homogeneously inthe matrix of hydroxypropylated starch. There is no observable change of both particle sizeand brightness for the cross-linked starch; as the particles content of the cross-linked starchincrease from0%to25%, Young's modulus and tensile strength showed an maximumincrease of75%and44%respectively. As the phosphate content of the cross-linked starchincreased from0%to0.06%, the tensile strength and Young's modulus of the films increased significantly.