Polylactide as a valuable biodegradable polymer is being widely investigated with respect to its synthesis, physical properties, biodegradation and application. The aim of this study is to evaluate hydrolytic degradation of poly (l-lactide) (PLLA) in the presence of added l-lactide dimer. High molecular weight PLLA was synthesized in presence of stannous octoate through the ring opening polymerization of l-lactide. PLLA films, containing 0, 1, 3 and 5% (w/w) l-lactide (as additive), were prepared by solution casting. In vitro degradation of the PLLA matrices were carried out in distilled water at 37°C for the definite periods. The degraded polymer matrices have been characterized by SEC, SEM and DSC techniques after periods of 3 and 6 months degradation time. It was found that during the first 3 months of degradation period, the number average molecular weight (¯Mn) of each PLLA film containing l-lactide reduced slower than the control sample. Also, it is shown that the films containing l-lactide have higher crystallinity and melting point in comparison with non-containing l-lactide samples. However, after 6 months, degradation rate of PLLA matrices containing l-lactide increased due to penetration of water by eluting and removal of l-lactide from PLLA matrices.

This study investigates the thermal, morphological, mechanical, and biodegradation properties of blends comprising poly(L-lactide)-b-poly (ethylene glycol)-b-poly(L-lactide) triblock copolymer (PLLA-PEG-PLLA) and polypropylene (PP) compared to the PLLA/PP blends. All the blends were prepared using a melt-blending technique. The ability to crystallize and the thermal stability of the PLLA-PEG-PLLA were enhanced by blending PP, whereas these properties were not enhanced for the PLLA. The crystallinity of PLLA-PEG-PLLA increased from 20.4% to 23.5-38.8%. The temperature at maximum decomposition rate (Td,max) of PLLA end-blocks increased from 310 °C to 327-332 °C. The phase compatibility between the PLLA-PEG-PLLA and PP was better than between the PLLA and PP, as indicated by the smaller size of dispersed PP particles in the PLLA-PEG-PLLA matrix. The PLLA-PEG-PLLA/PP blends showed greater flexibility, higher wettability, and faster biodegradation rates than the PLLA/PP blends. This suggests that these PLLA-PEG-PLLA/PP blends may be used as flexible and partially biodegradable plastics.

The effects of magnesium hydroxide and l-lactide dimer as additives on thermal properties and morphology of poly (l-lactide) (PLLA) films were studied. Hence, neat PLLA films and films containing additives (10% w/w) were prepared in dichloromethane at room temperature via solution casting. To evaluate thermal history on polymer properties, PLLA films were annealed by different processes. In one process, designated as A, PLLA films were heated from 20 up 140oC, then held for 1 h and cooled to room temperature. In another process, designated as B, melted PLLA samples which had been maintained at 200oC for 5 min were cooled with a rate of - 20oC/min to 140oC and annealed for 1 h before being cooled to room temperature. In the next process designated as C, the melted PLLA samples after 5 min of staying at 200oC were being quenched to 0oC. Then the samples were heated to 140oC followed by annealing for 1 h before being cooled to room temperature. The crystallization and morphology properties of PLLA films were studied using polarized optical microscopy and differential scanning calorimetry. It is found that the percentage of crystal and spherulitic formation inside PLLA films were influenced by thermal history and presence of the additives. The type of additives did not affect melting point (Tm) of the films annealed through different processes of A to C, while they had influence on Tm of the films, significantly. It is to be noted that some spherulites were formed in films during B process. During C process, however, the nucleation rate increased due to quenching which enhanced the spherulites formation.

Poly (L-lactide) is a chiral crystalline and biodegradable polymer. High molecular weight poly (L-lactide) is preferred in the production of surgical implant for internal fixation. These polymers are usually synthesized from L-lactide (cyclic di-ester of L-lactic acid). For the production of poly (L-lactide), bulk polymerization is the preferred procedure, but high temperature of the reaction may cause the thermal degradation of polymer. Therefore, optimizing the temperature and the duration time of the reaction for synthesis of high molecular weight polymer are very important. It is found that polymerization time and maximum molecular weight increase with decreasing reaction temperature, but when the reaction temperature is above the melting point of polymers, degradation is more extensive and faster, therefore, when the reaction temperature is set under the melting point of polymer the maximum molecular weight and the polymerization time are slightly changed. Glass transition temperature (Tg) and melting point (Tm) increased with increasing molecular weight. Tg and Tm of obtained polymers are 65-69°C and 160-180°C, respectively.

The physical and mechanical properties of poly (l-lactide) /poly (e-caprolactone) (PLLA/PCL) blends reinforced with multiwalled carbon nanotubes (MWCNTs) before and after in vitro degradation were investigated. Because of brittleness, PLLA needs to be plasticized by PCL as a soft polymer. The MWCNTs are used to balance the stiffness and the flexibility of PLLA/PCL blends. The results showed that with incremental increase in concentration of MWCNTs in composites, the agglomerate points of MWCNTs were increased. The physical and mechanical properties of prepared PLLA/PCL blends and MWCNT/PLLA/PCL nanocomposites were characterized. The X-ray diffraction analysis of the prepared blends and composites showed that MWCNTs, as heterogeneous nucleation points, increased the lamella size and therefore the crystallinity of PLLA/PCL. The mechanical strength of blends was decreased with incremental increase in PCL weight ratio. The mechanical behavior of composites showed large strain after yielding and high elastic strain characteristics. The tensile tests results showed that the tensile modulus and tensile strength are significantly increased with increasing the concentration of MWCNTs in composites, while, the elongation-at-break was decreased. The in vitro degradation rate of polymer blends in phosphate buffer solution (PBS) increased with higher weight ratio of PCL in the blend. The in vitro degradation rate of nanocomposites in PBS increased about 65% when the concentration of MWCNTs increased up to 3% (by weight). The results showed that the degradation kinetics of nanocomposites for scaffolds can be engineered by varying the contents of MWCNTs.

Controlling the rate and behaviour of the biodegradable polymer matrix is important in the development of drug delivery systems. In this project, we succeeded to control the speed of degradation and changing the degradation site from the bulk to the surface by addition of excipients. Antiacid excipients, such as Mg (OH)2 have significant effects on rate and behaviour of biodegradation, by neutralization of the acidic microclimate pH in polymer. We synthesized high molecular weight poly (L-lactide) by using tin- 2-ethyl hexanoate as catalyst. The polymer has been characterized by GPC, DSC and SEM. Mixtures of the polymer with Mg (OH) 2 at 1, 3 and 5% w/w were prepared and the degradation of the samples, kept at in vitro condition after 3 and 6 months, were studied. The results of average molecular weight changes, thermal characteristics and morphology of samples after 3 and 6 months revealed that it is possible to redirect the bulk degradation towards surface degradation. It is found that the ratio of bulk degradation to surface degradation and speed of degradation has reverse relationship with the additives concentration and when Mg (OH)2 increases, the speed of degradation decreases as well. In samples with Mg(OH)2, the polymer degradation rates were reduced by 2-3 folds and the percentage of crystallinity increased by maximum 90% (3% Mg(OH)2) after 6  months.