Controlling the microstructure of a random copolymer of poly (lactic acid) and its composite with hydroxyapatite for bone repair

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Salary, Zeinab (2020). Controlling the microstructure of a random copolymer of poly (lactic acid) and its composite with hydroxyapatite for bone repair. University of Birmingham. Ph.D.

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Abstract

Poly (lactic acid) or PLA and its composites have been extensively used for different biomedical applications. However, there has been limited research in controlling the microstructure and crystallisation behaviour of PLA as the microstructure of the polymer could have an effect on the performance of the material.

In this study, the microstructure and crystallinity of a semi-crystalline random copolymer of poly (L-lactic acid) with 4% D-isomer were controlled by using Differential Scanning Calorimetry (DSC) and Hot Stage Microscopy (HSM) under both nonisothermal and isothermal conditions. From the nonisothermal crystallisation studies, it was found that by controlling the cooling rate of the polymer after melting, polymers with different microstructures and degree of crystallinity can be obtained. Additionally, according to the isothermal studies, increasing the isothermal temperature resulted in an increase in the size of spherulites, while their number decreased. The n values obtained from the Avrami model was around 3, indicating a three-dimensional crystal growth mechanism with spherical crystals geometry. In addition, PLA and hydroxyapatite (HA) composites were prepared by using three different procedures; hot pressing, injection moulding and solution casting in order to improve the bioactivity of the polymer and investigate the effect of HA particles on the crystallisation behaviour of the composite. The hot pressing preparation method was the most effective technique in terms of blending the PLA and HA particles homogeneously. According to DSC, HSM studies and Avrami analysis of the crystallisation of the composite materials, the HA particles acted as nucleating agents to facilitate the crystallisation process of PLA. Mechanical testing results revealed that the stiffness of the composites increased with increasing the HA content. Dichloromethane (DCM) was also used to etch the surface of PLA samples in order to introduce porosity by etching the surfaces for various periods of time. Pores were formed on the surfaces of the PLA samples evidenced by scanning electron microscopy (SEM) and therefore etching could be an easy and simple way of adding porosity on the polymer surfaces aiming to better interaction of the surfaces with mammalian cells.

The effect of different spherulitic microstructures on the nano-hardness of PLA surfaces was investigated. The values of nano-hardness and Young’s modulus of the PLA samples containing larger spherulites were higher (0.377and 7.79 GPa, respectively) in comparison with the samples containing smaller spherulites (0.343 and 7.31GPa, respectively) due to the increase in the thickness of lamellae of the samples containing larger spherulites. Furthermore, the effect of secondary crystallisation on the wear behaviour of PLA samples was explored. It was found that the amount of mass loss was less in a semi-crystalline sample compared to an amorphous one. However, by introducing the secondary crystallisation, wear resistance of the PLA samples was reduced.

Hydrolytic degradation studies were performed on PLA samples under both static and dynamic conditions. By increasing the temperature of the degradation medium, the hydrolysis rate and mass loss were increased under static conditions. However, the mass of all samples did not change under dynamic conditions because the pH was almost unchanged during the experiment due to the circulation of the aqueous environment. According to the Size Exclusion Chromatography (SEC) results of the samples under dynamic conditions, the PLA samples with the small size of spherulites showed a smaller change in the molecular weight compared to the samples with the larger size of PLA spherulites, suggesting a possible increase of stability of PLA with smaller spherulites size in reaction with the buffer. Furthermore, introducing HA into PLA samples would increase the stability of the samples in the buffer due to the alkaline effect of HA particles that leads to neutralisation of the acidic degradation products of PLA. DSC results revealed that the degree of crystallinity of all PLA and PLA-HA samples increased significantly after the hydrolytic degradation under dynamic conditions. Additionally, according to the X-ray Diffraction (XRD) results, the amorphous PLA sample crystallised during hydrolytic degradation.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):